It all began with a tip: In the aftermath of the July 7, 2005, subway bombings in London, British authorities received a call from a worried member of the Muslim community, reporting general suspicions about an acquaintance.

From that vague but vital piece of information, according to a senior European intelligence official, British authorities opened the investigation into what they said turned out to be a well-coordinated and long-planned plot to bomb trans-Atlantic flights heading toward the United States — an assault designed to rival the scope and lethality of the Sept. 11, 2001 hijackings.

The 24 British nationals arrested yesterday were in the final stages of an alleged plan to detonate liquid explosives on as many as 10 U.S.-bound airliners, British authorities said.

"Quite simply, this was intended to be mass murder on an unimaginable scale," said Paul Stephenson, deputy chief of London's Metropolitan Police.

The Bank of England froze the assets of 19 people early today, naming them as people arrested in connection with an alleged terror plot.

"On the advice of the police and security services, the Treasury has instructed the Bank of England to issue notices to effect a freeze of the assets of a number of individuals arrested in yesterday's operations," a Treasury statement said.

Officials could not say when the attacks were to be carried out, but said the suspects, mainly of Pakistani descent, had acquired many of the materials to assemble explosives that could have killed hundreds of airline passengers as their planes crossed the Atlantic Ocean from British airports.

Starting today, passengers will be required to submit to secondary screening of their carryon items at aircraft gates, in addition to the screening at the main security checkpoints. The expanded gate searches were implemented after the 2001 attacks, and later eliminated.

The arrests, made in Birmingham, London and High Wycombe, a commuter town west of the capital, were quickly followed by flight cancellations and an extreme security crackdown at nearly all major American and British airports, leading to delays and chaos throughout the world's commercial airline system.

U.S. officials barred anything containing liquids or gels from carry-on luggage, while British airports banned all carry-on luggage, as well as laptop computers, cell phones and portable music devices.

The plotters reportedly planned to smuggle chemicals aboard flights by hiding them in everyday items such as beverage containers, then assemble them into an explosive mixture and ignite or detonate them using common electronic devices, investigators said.

The names released by the Bank of England included: Abdula Ahmed Ali, Cossor Ali, Shazad Khuram Ali, Nabeel Hussain, Tanvir Hussain, Umair Hussain, Umar Islam, Waseem Kayani, Assan Abdullah Khan, Waheed Arafat Khan, Osman Adam Khatib, Abdul Muneem Patel, Tayib Rauf, Muhammed Usman Saddique, Assad Sarwar, Ibrahim Savant, Amin Asmin Tariq, Shamin Mohammed Uddin and Waheed Zaman.

The oldest person on the list, Shamin Mohammed Uddin, is 35. The youngest, Abdul Muneem Patel, is 17.

INVESTIGATION WIDENS

By late 2005, the probe by British authorities had expanded to involve several hundred investigators on three continents who kept dozens of suspects under close surveillance for months, even as some of the plotters traveled between Britain and Pakistan to raise money, find recruits and refine their scheme, according to interviews with U.S. and European counterterrorism officials.

Precise details of the plot — how many planes, their destinations and the date — remain unknown. The shape of the operation changed regularly as the would-be bombers considered which trans-Atlantic flights to target and prepared for a practice run, which was expected to take place in the next few days, U.S. law-enforcement officials said.

Investigators eventually pieced together enough information from a blizzard of stakeouts, tips and wiretaps to make clear that something big was in the works and that the plotters' preparations were nearing an end.

"It's not like three weeks ago all of a sudden MI5 knew about this plot and went to work," a U.S. law enforcement official said, speaking of the British security service. "They'd had a concern about these guys for some time, for months. Details started to emerge, and it became clear over the last couple weeks the nature of the threat and the individuals," said the official, who like others interviewed for this story spoke on condition of anonymity.

A law-enforcement bulletin issued yesterday by the Department of Homeland Security and the FBI described the conspiracy as "international in scope" but said there was no evidence that the plotters or any accomplices had set foot in the United States. "This plot appears to have been well-planned and well-advanced and in the final stages of preparation," the bulletin stated.

One U.S. intelligence source, however, said some of the British suspects arrested had made calls to the United States.

In Pakistan, a senior government official said two British nationals of Pakistani origin had been arrested, one in Lahore and one in Karachi, about a week ago. The two provided information on the terror plot, the official said on condition of anonymity because he did not have authority to speak formally on the issue.

THREE CELLS

Investigators believe the London operation was composed of three distinct cells, whose members may not have been aware of one another or the extent of their assignment, U.S. officials said.

British officials suspect that as many as 50 participants and accomplices were involved, U.S. law enforcement officials said. Internet searches made by the suspects suggested that they had considered targeting up to 10 different flights, investigators said, although there was no evidence that those arrested had bought tickets or made reservations.

"It's fair to say they were aiming for multiple flights, and some of the exact data of who they would deploy and how many might be in one deployment are somewhat ambiguous," said Michael Jackson, deputy secretary at the Department for Homeland Security. "There were different data sets about their intentions over time, that evolved over the period of time that we were following this. It did seem in more recent days to have centered upon carriers that had direct, nonstop flights between the U.K. and U.S.

"The real focus was to blow up airliners and the people on them."

Counterterrorism officials said the basic outline of the conspiracy was known for several months. Investigators from New York to Islamabad said they were briefed by their British counterparts late last year.

British and U.S. law-enforcement authorities decided against breaking up the cells right away in hopes that they could learn more about the origins of the network and assemble evidence for prosecutors.

Some U.S. counterterrorism officials said plans originally were to allow the conspiracy to develop even further. But U.S. and British investigators suddenly decided this week to close down the operation after becoming increasingly worried that there were other bombers they had failed to locate or identify, U.S. officials said.

British Home Secretary John Reid said that "the police are confident that the main players have been accounted for" and were in custody. But U.S. and European authorities said the widespread ban against carrying liquids onto flights was imposed because investigators were worried more conspirators could be at large.

The Los Angeles Times, Associated Press and Chicago Tribune contributed to this report.