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.

Another British official stressed that martyrdom videos were often made well in advance of an attack. In fact, two and a half weeks since the inquiry became public, British investigators have still not determined whether there was a target date for the attacks or how many planes were to be involved. They say the estimate of 10 planes was speculative and exaggerated.

In his first public statement after the arrests, Peter Clarke, chief of counterterrorism for the Metropolitan Police, acknowledged that the police were still investigating the basics: “the number, destination and timing of the flights that might be attacked.”

A total of 25 people have been arrested in connection with the suspected plot. Twelve of them have been charged. Eight people were charged with conspiracy to commit murder and preparing acts of terrorism. Three people were charged with failing to disclose information that could help prevent a terrorist act, and a 17-year-old male suspect was charged with possession of articles that could be used to prepare a terrorist act. Eight people still in custody have not been charged. Five have been released. All the suspects arrested are British citizens ranging in age from 17 to 35.

Despite the charges, officials said they were still unsure of one critical question: whether any of the suspects was technically capable of assembling and detonating liquid explosives while airborne.

A chemist involved in that part of the inquiry, who spoke on the condition of anonymity because he was sworn to confidentiality, said HMTD, which can be prepared by combining hydrogen peroxide with other chemicals, “in theory is dangerous,” but whether the suspects “had the brights to pull it off remains to be seen.”

While officials and experts familiar with the case say the investigation points to a serious and determined group of plotters, they add that questions about the immediacy and difficulty of the suspected bombing plot cast doubt on the accuracy of some of the public statements made at the time.

“In retrospect,’’ said Michael A. Sheehan, the former deputy commissioner of counterterrorism in the New York Police Department, “there may have been too much hyperventilating going on.”

Some of the suspects came to the attention of Scotland Yard more than a year ago, shortly after four suicide bombers attacked three subway trains and a double-decker bus in London on July 7, 2005, a coordinated attack that killed 56 people and wounded more than 700. The investigation was dubbed “Operation Overt.’’

The Police Are Tipped Off

The police were apparently tipped off by informers. One former British counterterrorism official, who was working for the government at the time, said several people living in Walthamstow, a working-class neighborhood in East London, alerted the police in July 2005 about the intentions of a small group of angry young Muslim men.

Walthamstow is best known for its faded greyhound track and the borough of Waltham Forest, where more than 17,000 Pakistani immigrants live in the largest Pakistani enclave in London.

Armed with the tips, MI5, Britain’s domestic security services, began an around-the-clock surveillance operation of a dozen young men living in Walthamstow — bugging their apartments, tapping their phones, monitoring their bank transactions, eavesdropping on their Internet traffic and e-mail messages, even watching where they traveled, shopped and took their laundry, according to senior British officials.

The initial focus of the investigation was not about possible terrorism aboard planes, but an effort to see whether there were any links between the dozen men and the July 7 subway bombers, or terrorist cells in Pakistan, the officials said.

The authorities quickly learned the identity of the man believed to have been the leader of the cell, the unemployed man in his mid-20’s, who traveled at least twice within the past year to Pakistan, where his activities are still being investigated.

Last June, a 22-year-old Walthamstow resident, who is among the suspects arrested Aug. 10, paid $260,000 cash for a second-floor apartment in a house on Forest Road, according to official property records. The authorities noticed that six men were regularly visiting the second-floor apartment that came to be known as the “bomb factory,” according to a British official and the person briefed about the case.

Two of the men, who were likely the bomb-makers, were conducting a series of experiments with chemicals, said the person briefed on the case.

MI5 agents secretly installed video and audio recording equipment inside the apartment, two senior British officials said. In a secret search conducted before the Aug. 10 raids, agents had discovered that the inside of batteries had been scooped out, and that it appeared several suspects were doing chemical experiments with a sports drink named Lucozade and syringes, the person with knowledge of the case said. Investigators have said they believe that the suspects intended to bring explosive chemicals aboard planes inside sports drink bottles.

In that apartment, according to a British official, one of the leaders and a man in his late 20’s met at least twice to discuss the suspected plot, as MI5 agents secretly watched and listened. On Aug. 9, just hours before the police raids occurred in 50 locations from East London to Birmingham, the two men met again to discuss the suspected plot and record a martyrdom video.

As one of the men read from a script before a videocamera, he recited a quotation from the Koran and ticked off his reasons for the “action that I am going to undertake,” according to the person briefed on the case. The man said he was seeking revenge for the foreign policy of the United States, and “their accomplices, the U.K. and the Jews.” The man said he wanted to show that the enemies of Islam would never win this “war.”

Beseeching other Muslims to join jihad, he justified the killing of innocent civilians in America and other Western countries because they supported the war against Muslims through their tax dollars. They were too busy enjoying their Western lifestyles to protest the policies, he added. Though British officials usually release little information about continuing investigations, Scotland Yard took the unusual step of disclosing some detailed information about the investigation last Monday, when the suspects were charged.

A Trove of Evidence

“There have been 69 searches,” Mr. Clarke, the chief antiterrorist police official from Scotland Yard, said Monday. “These have been in houses, flats and business premises, vehicles and open spaces.”

Investigators also seized more than 400 computers, 200 mobile phones and 8,000 items like memory sticks, CD’s and DVD’s. “The scale is immense,” Mr. Clarke said. “Inquiries will span the globe.”

He said those searches revealed a trove of evidence, and officials and others last week provided additional details.

Four of the law firms that are defending suspects declined to comment.

When police officers knocked down the door to the second-floor apartment on Forest Road, they found a plastic bin filled with liquid, batteries, nearly a dozen empty drink bottles, rubber gloves, digital scales and a disposable camera that was leaking liquid, the person with knowledge of the case said. The camera might have been a prototype for a device to smuggle chemicals on the plane.

In the pocket of one of the suspects, the police found the computer memory stick that showed he had looked up airline schedules for flights from London to the United States, a British official said. The man is said to have had a diary that included a list that the police interpreted as a step-by-step plan for an attack. The items included batteries and Lucozade bottles. It also included a reminder to select a date.

In the homes of a number of the suspects, the police found jihadist literature and DVD’s about “genocide” in Iraq and Palestine, according to British officials. In one house searched by the police in Walthamstow, the authorities found a copy of a book called “Defense of the Muslim Lands.”

A “last will and testament” for one of the accused was said to have been found at his brother’s home. Dated Sept. 24, 2005, the will concludes, “What should I worry when I die a Muslim, in the manner in which I am to die, I go to my death for the sake of my maker.” God, he added, can if he wants “bless limbs torn away!!!”

Looking for Global Ties

In addition, the British authorities are scouring the evidence for clues to whether there is a global dimension to the suspected plot, particularly the extent to which it was planned, financed or supported in Pakistan, and whether there is a connection to remnants of Al Qaeda. They are still trying to determine who provided the cash for the apartment and the computer equipment and telephones, officials said.

Several of the suspects had traveled to Pakistan within weeks of the arrests, according to an American counterterrorism official.

At a minimum, investigators say at least one of the suspects’ inspiration was drawn from Al Qaeda. One of the suspects’ “kill-as-they-kill” martyrdom video was taken from a November 2002 fatwa by Osama bin Laden.

British officials said many of the questions about the suspected plot remained unanswered because they were forced to make the arrests before Scotland Yard was ready.

The trigger was the arrest in Pakistan of Rashid Rauf, a 25-year-old British citizen with dual Pakistani citizenship, whom Pakistani investigators have described as a “key figure” in the plot.

In 2000, Mr. Rauf’s father founded Crescent Relief London, a charity that sent money to victims of last October’s earthquake in Pakistan. Several suspects met through their involvement in the charity, a friend of one of them said. Last week, Britain froze the charity’s bank accounts and opened an investigation into possible “terrorist abuse of charitable funds.” Leaders of the charity have denied the allegations.

Several senior British officials said the Pakistanis arrested Rashid Rauf without informing them first. The arrest surprised and frustrated investigators here who had wanted to monitor the suspects longer, primarily to gather more evidence and to determine whether they had identified all the people involved in the suspected plot.

But within hours of Mr. Rauf’s arrest on Aug. 9 in Pakistan, British officials heard from intelligence sources that someone connected to him had tried to contact some of the suspects in East London. The message was interpreted by investigators as a possible signal to move forward with the plot, officials said.

“The plotters received a very short message to ‘Go now,’ ” said Franco Frattini, the European Union’s security commissioner, who was briefed by the British home secretary, John Reid, in London. “I was convinced by British authorities that this message exists.”

A senior British official said the message from Pakistan was not that explicit. But, nonetheless, investigators here had to change their strategy quickly.

“The aim was to keep this operation going for much longer,” said a senior British security official who requested anonymity because of confidentiality rules. “It ended much sooner than we had hoped.”

From then on, the British government was driven by worst-case scenarios based on a minimum-risk strategy.

British investigators worried that word of Mr. Rauf’s arrest could push the London suspects to destroy evidence and to disperse, raising the possibility they would not be able to arrest them all. But investigators also could not rule out that there could be an unknown second cell that would try to carry out a similar plan, officials said.

Mr. Clarke, as the country’s top antiterrorism police official in London with authority over police decisions, ordered the arrests.

But it was left to Mr. Reid, who has been home secretary since May and is a former defense secretary, to decide at emergency meetings of police, national security and transport leaders, what else needed to be done. Mr. Reid and Mr. Clarke declined repeated requests for interviews.

Prime Minister Tony Blair was on vacation in Barbados, where he was said to have monitored events in London; Deputy Prime Minister John Prescott did not attend the meeting.

“While the arrests were unfolding, the Home Office raised Britain’s terror alert level to “critical,” as the police continued their raids of suspects’ homes and cars. All liquids were banned from carry-on bags, and some public officials in Britain and the United States said an attack appeared to be imminent. In addition to Mr. Stephenson’s remark that the attack would have been “mass murder on an unimaginable scale,” Mr. Reid said that attacks were “highly likely” and predicted that the loss of life would have been on an “unprecedented scale.”

Two weeks later, senior officials here characterized the remarks as unfortunate. As more information was analyzed and the British government decided that the attack was not imminent, Mr. Reid sought to calm the country by backing off from his dire predictions, while defending the decision to raise the alert level to its highest level as a precaution.

In lowering the threat level from critical to severe on Aug. 14, Mr. Reid acknowledged: “Threat level assessments are intelligence-led. It is not a process where scientific precision is possible. They involve judgments.”