cnn's international diplomatic editor nic robertson is on the seen in salisbury, where the poisoning occurred. what's the latest? >> reporter: wolf, right behind me is the house sergei skripal was living in. it's a potential danger to citizens living around here. it has a permanent police cordon. it's still part of an active crime scene here. what the police seem to have discovered could be a very helpful and useful lead in all of this. a source familiar with the investigation has told cnn that the british authorities were able to pick up a coded russian transmission to moscow soon after the attack that said the two attackers had left the country. what police have been able to do after that is sort of work backwards from that, take all the close circuit security