Tributes are being paid to the victims of monday nights attack. The suspect, salman abedi, a 22 year old born in manchester to libyan parents had been known to Security Services. It is said it is unlikely he was acting alone. The Terror Threat alert has been raised to critical, its highest level. More rate of taken place across the city today as Police Continue their investigation. So far, four people have been arrested, one is the suspects older brother. The bbc understands the bomber is thought to have detonated a device built by someone else. The Terror Threat alert indicates further attacks may be imminent. Hundreds of troops have been deployed to protect key sites across the country, including Downing Street. An Off Duty FemalePolice Officer was among the 22 killed in the attack. Husband is critically ill and their two children were also injured. We will have all the latest elements in manchester and reaction, but first this report. Of those killed in the attack on monday night, more have now been named, including olivia campbell, who was 15 years old. Her family have been desperately searching for her, hoping she was still alive. And martyn hett, a pr manager, whose friends described him as wonderful, iconic and beautiful. This afternoon the Greater Manchester police chief main announcement about one of his officers. Very sadly i can confirm that one of the victims is a serving Police Officer. In respecting their familys wishes i will make no further comment at this stage. The Investigation Continues at pace and four men have been arrested. The police are confirmed they are investigating a network of extremists. They believe it is very unlikely that abedi was working alone. He was brought up in manchester. He has been described as gullible and someone who turned to religion five years ago. It is nabi discovered he has recently travelled to countries where islamic extremists are based. Todayit today it was revealed that dangers we re today it was revealed that dangers were flagged up years ago. The decision was taken to raise the Terrorist Threat level to critical. This means another attack could be in and. It gives the authorities the power to deploy thousands of troops across the country. Some are already in place, guiding Downing Street, whitehall, and around westminster. Whilst we are at critical and there is the possibility of a further attack, we want to be able to support the public as best we can and protect them as best we can, and we believe we need more armed officers on the streets. Submitting military colleagues and what i would call static posts in this case, the palace of westminster, allows us to put Police Officers with firearms in greater numbers across the uk. Put Police Officers with firearms in greater numbers across the Ukm Manchester while people continue to grieve, there is also determination to remain united as a community. Grieve, there is also determination to remain united as a communitym was our darkest hour but also you saw the best of greatest manchester yesterday. I was in the Hospital S Late Last Night and i was Hearing Stories that porters, cleaners, surgeons, nurses, came stories that porters, cleaners, surgeons, nurses, came in off shift to help out. People brought in food. Everybody pulled together and we should take a great deal of pride in that, that at our most testing moment Greater Manchester came together. That sense of Community Spirit is vital but it will still ta ke spirit is vital but it will still take time for this city to recover from the trauma of what happened here on monday night. We will take you straight to saint anne s square in manchester. Thank you. Indeed it is a very sunny day in manchester but the atmosphere as you can imagine is solomon and sombre. This is now the focal point of the citys expressions of simply and solidarity. We were in Albert Square earlier and the tributes have now been moved here. There is of course an Important Development potentially in our reporting today, and that is the reporting of the possibility, as we understand it, of warnings having been delivered to the police and to the Intelligence Services as long ago as five years will stop about the extreme and violent abuse which are attributed to salman abedi, the suspected bomber who was responsible for those 22 deaths and many injuries on monday night at the Manchester Arena. I think it is worth telling you a little more about what were being told, because we are being told Muslim Community worker reports members of the public called the police and Anti Terrorist hotline warning about the views of abedi several years ago. We understand thatis several years ago. We understand that is five years ago. The Community Worker who did not want to be identified said that two people who knew abedi at college has made separate calls to the police at that time, they had been worried he was supporting terrorism, that is the quote, and expressed the view that being a Suicide Bomber was something he considered to be a valid thought. The friends had argued with him, they said, telling him he was wrong, but they have become so concerned that they then came into contact with police. The Community Worker said all the publicity these days is about muslims not coming forward, and this statement was being made to prove that some had come forward in the past and had expressed their concerns. Then the Community Worker goes on to say, this shows the present policy is a failure. The prevent policy is the official Government Strategy to counter the radicalisation of especially young people. This Community Worker says prevent is a failure because it did not work, making lots of moslems feel scared does not work and it did not stop this person being radicalised. That is the information the bbc got this afternoon from a local Muslim Community worker, reporting that these warnings have been made as long ago as 2012, 2011. More information has been emerging about the suspected bomber. He was 22, born in manchester, his parents we re 22, born in manchester, his parents were libyans, 22, born in manchester, his parents were libya ns, refugees. 22, born in manchester, his parents were libyans, refugees. At one point he was a student at Salford University and the fellow students i have just quoted were there at the same time. It is reported he recently travelled to libya, returned to the uk just a few days before the attack. We are also being told by French Security officials that it told by French SecurityOfficials Thatitis told by French Security officials that it is likely in their view that he did Notjust Travel To Libya but also syria as well. We are not able to confirm that at this point but thatis to confirm that at this point but that is the latest briefing from the french Intelligence Services. We will talk more about this but clearly, potentially, it is an Important Development in the way the story is reported. Lets have a report from our security correspondent. The face of salman abedi. Identified as a Manchester City fire bomber. Hes now thought to have had helped quite possibly from an expert bomb maker. This Amateur Footage taken yesterday is believed to show his brother being arrested in manchester. At the family home nearby, police used explosives to gain access and then search for clues. But there are International Connections to this plot. From manchester, abedi is believed to have gone to libya, a failed state. And today, the French Interior minister said war torn syria was also a country hed probably spent time in. Increasingly, investigators have been coming round to the conclusion that the bomber, salman abedi, was not working alone. That he was simply the mule, carrying a device designed and built for him by somebody else. Now they are talking about him being pa rt now they are talking about him being part of a terror network. M15, the security service, is working with police and other Intelligence Agencies to try to trace every possible connection he had in this country and abroad. This is what we know about salman abedi. He was born in manchester in 1994 of libyan parents from tripoli. He dropped out of Salford University and was already known to Security Services. In fact, the individual i spoke to yesterday who was a very close friend of the family, described him as a nieve individual. Described him as a naive individual. It would indicate he could not have put the explosive belt on. He must have had support helping him do that. Meanwhile, the search for clues continues. Forensic, digital and testimonies from witnesses and suspects. This, say the police, is a fast moving, wide ranging investigation. Lets talk more about the investigation, wide ranging and fast moving. Extensive and now international as well. 0ur correspondent Catriona Renton is at Greater Manchester police headquarters. What are you being told theyre about the likely direction of this investigation . As you said the investigation . As you said the investigation is moving at quite a pace. Let me bring you up to date with what we have learned so far. It appears a crucial question in this investigation may have now been answered. Was Salman Abedi Acting alone . The Chief Constable spoke here earlier today and he said it is very clearly a network that we are investigating. We also know that four men are currently in custody. 0vernight there were three arrests in south manchester, men believed to have links to an address where salman abedi used to live. And yesterday his 23 year old Brother Ismail was also arrested. None of these men has been charged. This Afternoon Police also raided a Block Of Flats near manchester piccadilly station. A controlled explosion was carried out and for a short wail and railway line was closed. They were asked today with a bomb factory had been discovered. The Police Answered that extensive searches had been carried out. It is crucial now that the police find where this bomb was made, until they find this out they do not know if there are other bombs, other bombers. So they need to find out who made the bomb, did Anyone Encourage Salman Abedi to carry out the horrific attack on monday evening . Also today Police Confirmed that an off duty Police Officer was among those killed. The bbc understands she was from the cheshire force. Her husband is critically ill in hospital and their two children have also been injured. They also said that all the victimss families have been informed of what has happened to them and Specialist Police are supporting those families of those who sadly died. In terms of keeping the city going, there is a very visible Police Presence today. We have seen armed Police Patrolling round here throughout the day. The Chief Constable has said no Military Personnel are here in manchester, but what is happening in the army being able to free up officers means things like the Manchester Games and the Manchester Ten K can go ahead, trying to keep some semblance of normality in this city while the Investigation Continues. Thank you. Lets talk more broadly about the response we have had across all agencies and in government to the attack. Joining me now from westminster is the chair of the intelligence and Security Committee and former attorney general dominic grieve. It is good to have you with us. Thinking of the increase of the Terror Threat level to its maximum, troops being deployed in major cities, can we have your assessment of the way the response has been effected . Of the way the response has been effected . It must be the right response on the basis of the police, and the Security Services believing there is a network and therefore a possibility of that network launching a fresh attack. So from that Point Of View the response appears to me to be exactly what i would expect. And when we look at the fact that of course the deployment of troops during an election campaign, for some people, is maybe understandable but also in democratic terms a worrying thing. What are your thoughts on that . They are there to protect the democratic process. If violence is taking place that undermines democracy. And our freedoms. It is important that we should be able to complete this general election peacefully. It certainly does not interfere with campaigning in anyway and when the right moment comes the campaigning will resume. As someone who has chaired the intelligence and Security Committee what kind of questions would you be asking at this point about the way things are dialling . I think everything i have seen so far suggests that the way the enquiry is being conducted is what i would expect and i appreciate listening to the earlier comments that there are some questions being raised about whether this man came to the attention of the Security Services earlier. And that is obviously something that will be of interest subsequently in finding out exactly what happened. It is of course very different for the Security Services if they get such a reference to them, they have to make out an assessment about is this someone who is likely to pose a risk or is actively getting involved in a Terrorist Network or in some form of planning. So the mere fact that reference is made does not necessarily mean that several years down the line he would have to be a subject of interest. It all has to be inevitably prioritised according the Security Services view of the greatest risk. We mentioned earlier that one of the brothers of the suspect was arrested in manchester yesterday, and we are being told this afternoon that the younger brother has been arrested in tripoli today on suspicion of links with islamic state. I am wondering for some of your thoughts, cautious thoughts i know, about the libyan context thoughts i know, about the libyan co ntext a nd thoughts i know, about the libyan context and whether that should maybe have been flagged up more prominently in recent years. Maybe have been flagged up more prominently in recent yearslj maybe have been flagged up more prominently in recent years. I find that a very difficult question to answer. You have to remember the family were as i understand refugees from gaddafis libya. Admittedly libya is now in a pretty chaotic state but it is no indication of a link to dyche extremism if somebody decides to go back to libya After The Fall of gaddafi to visit relatives. The fact he may have gone to syria is another matter, but in itself i would not necessarily infer that because someone has a libyan connection it means they must be linked to terrorism. Mr grieve, good of you tojoin linked to terrorism. Mr grieve, good of you to join us. Its known that 22 people were killed in the bomb attack, the names of 1a victims have now been revealed. Greater Manchester Police have confirmed one of their off Duty Officers is among the dead, although they did not reveal her identity. The bbc understand the officers husband is critically ill, and their two children are also injured. Tributes have been paid to all the victims, including eight year old Saffie Roussos, described by her headteacher as simply a beautiful little girl in every aspect of the word. Lots of these