LONDON, Aug. 27 — On Aug. 9, in a small second-floor apartment in East London, two young Muslim men recorded a video justifying what the police say was their suicide plot to blow up trans-Atlantic planes: revenge against the United States and its “accomplices,” Britain and the Jews.

“As you bomb, you will be bombed; as you kill, you will be killed,” said one of the men on a “martyrdom” videotape, whose contents were described by a senior British official and a person briefed about the case. The young man added that he hoped God would be “pleased with us and accepts our deed.”

As it happened, the police had been monitoring the apartment with hidden video and audio equipment. Not long after the tape was recorded that day, Scotland Yard decided to shut down what they suspected was a terrorist cell. That action set off a chain of events that raised the terror threat levels in Britain and the United States, barred passengers from taking liquids on airplanes and plunged air traffic into chaos around the world.

The ominous language of seven recovered martyrdom videotapes is among new details that emerged from interviews with high-ranking British, European and American officials last week, demonstrating that the suspects had made considerable progress toward planning a terrorist attack. Those details include fresh evidence from Britain’s most wide-ranging terror investigation: receipts for cash transfers from abroad, a handwritten diary that appears to sketch out elements of a plot, and, on martyrdom tapes, several suspects’ statements of their motives.

But at the same time, five senior British officials said, the suspects were not prepared to strike immediately. Instead, the reactions of Britain and the United States in the wake of the arrests of 21 people on Aug. 10 were driven less by information about a specific, imminent attack than fear that other, unknown terrorists might strike.

The suspects had been working for months out of an apartment that investigators called the “bomb factory,” where the police watched as the suspects experimented with chemicals, according to British officials and others briefed on the evidence, all of whom spoke on condition of anonymity, citing British rules on confidentiality regarding criminal prosecutions.

In searches during raids, the police discovered what they said were the necessary components to make a highly volatile liquid explosive known as HMTD, jihadist materials, receipts of Western Union money transfers, seven martyrdom videos made by six suspects and the last will and testament of a would-be bomber, senior British officials said. One of the suspects said on his martyrdom video that the “war against Muslims” in Iraq and Afghanistan had motivated him to act.

Investigators say they believe that one of the leaders of the group, an unemployed man in his 20’s who was living in a modest apartment on government benefits, kept the key to the alleged “bomb factory” and helped others record martyrdom videos, the officials said.

Hours after the police arrested the 21 suspects, police and government officials in both countries said they had intended to carry out the deadliest terrorist attack since Sept. 11.

Later that day, Paul Stephenson, deputy chief of the Metropolitan Police in London, said the goal of the people suspected of plotting the attack was “mass murder on an unimaginable scale.” On the day of the arrests, some officials estimated that as many as 10 planes were to be blown up, possibly over American cities. Michael Chertoff, the secretary of the Department of Homeland Security, described the suspected plot as “getting really quite close to the execution stage.”

But British officials said the suspects still had a lot of work to do. Two of the suspects did not have passports, but had applied for expedited approval. One official said the people suspected of leading the plot were still recruiting and radicalizing would-be bombers.

While investigators found evidence on a computer memory stick indicating that one of the men had looked up airline schedules for flights from London to cities in the United States, the suspects had neither made reservations nor purchased plane tickets, a British official said. Some of their suspected bomb-making equipment was found five days after the arrests in a suitcase buried under leaves in the woods near High Wycombe, a town 30 miles northwest of London.