reporter: the suspect or suspects still at large. a further attack may be imminent. reporter: the full resources of scotland yard and british counterterrorism forces engaged in an intense manhunt. there are many inquiries with hundreds of inspectors involved looking at cctv and bombs. the bomb itself did not go off which for law enforcement say great thing because the game finger print of the individual who made it. reporter: a british source tells cnn a timer was found on the device, though it s clear it was a crude bomb it was intended to cause much greater damage. we re only aware of one device. and that device examined by our experts. reporter: one source briefed on the investigation saysen initial assessment of the bomb indicates it highly likely to have taken tapt, an unstable explosive that packs a nasty punch. this video shows tapt from the