LONDON – The critical breakthrough in the hunt for the London bombers came on Monday night in a police video viewing suite.

Detectives involved in watching thousands of hours of film for glimpses of the terrorists had been given a “profile” based on a simple question posed for their guidance by senior officers.

The question was: What would the terrorists look like? The answer was that they would be young men, probably in their 20s and 30s, and they would be carrying rucksacks.

At 8 p.m. on Monday, on footage from a camera at King’s Cross station in central London, officers found images of four young men carrying bulky rucksacks, similar to those in which soldiers carry radios. One source observed: “It was like the infantry going to war, or like they were going on a hiking holiday.”

One of the faces was known. It belonged to Hasib Hussain, a 18-year-old man from Leeds whose family had called the police casualty bureau at around 10 p.m. last Thursday to say he had gone to London with friends and was missing and was a potential victim.

A West Yorkshire Police “family liaison officer” was appointed to the family. Subsequently, documents that were linked to him – including a driving license and cash cards – were found in the debris around the no. 30 bus in Tavistock Square.

He remained a potential victim but – after his face was seen with the others at King’s Cross, in footage timed at around 8:30 a.m. on July 7 – he is now suspected of being one of the first four suicide bombers in Britain.

It is now believed that two other men, aged 30 and 22, also died. It is thought they have been identified as suspected suicide bombers by comparing images from the CCTV and from personal property found at three of the bomb scenes – Tavistock Square, Liverpool Street, and Edgware Road. None of the men are related.

Documents belonging to the older man, who lived with a partner, were found at Edgware Road. Documents linked to the 22-year-old – Shehzad Tanweer, who lived with a brother and his father – were found at Liverpool Street, where he is thought to have died.

All three identified men were British, of Pakistani background, and from West Yorkshire. So far, the wreckage in the Piccadilly line tunnel outside King’s Cross is so appalling, with more than 1,000 body parts, that no identification of a fourth bomber, who is believed to have died in the blast, can be made.

There is no explanation as to why a bus was blown up. One theory is that the bus bomber was unable to get on the Northern line, the intended target, as the four fanned out in the four compass directions from King’s Cross. How the bombs were detonated is so far unknown.

The emergence of the first plot in Britain involving multiple suicide victims has deeply troubled security chiefs. One commented: “What is the probability of all four coming to London and sadly being victims of terrorist attacks? It’s nil. The issue now is getting back to normal. But how will normality look now, after this?”

Senior sources said a classic Al Qaeda operation would involve an experienced organizer who entered Britain to recruit bombers but left a day before they attacked. Flight records from the period before the bombs are being analyzed for anyone who may fit this profile. Inquiries are also trying to establish associates and supporters of the bombers.

One senior security source said: “We may have the bombers but where are the plotters and planners? Where is the person with the expertise to trigger it? And is the capability still out there? It could be anywhere.”