The trial was one of the most protracted and complex ever held for a terrorism case in Britain, after what Scotland Yard described as one of the largest-scale criminal investigations ever mounted here.

After three months of evidence, the case against the eight men  all British Muslims aged 24 to 30, and six with family roots in Pakistan  went to the jury in late July. A two-week holiday break ordered by the judge was followed by what appeared to have been an impasse of more than three weeks among jurors on the most serious charges, even after the judge, David Calvert-Smith, allowed the jury to reach verdicts with at least 10 of the 12 jurors in agreement.

By exactly that margin, the jurors on Monday returned guilty verdicts on the murder conspiracy charges against three men who prosecutors said had been at the heart of the plot: Abdulla Ahmed Ali, 27, a husband and father who studied computer systems engineering; Assad Sarwar, 28, a college dropout who learned how to make the liquid hydrogen peroxide bomb in Pakistan; and Tanvir Hussain, 27, who helped in the purchase of materials and in making the suicide videos.

The defendants planned to drain 17-ounce plastic sports drink bottles by puncturing a tiny hole in the bottom, prosecutors said, then refilling the bottles with an explosive mix of concentrated hydrogen peroxide and food coloring to give the appearance of the original beverage.

The prosecution said the bottles were to have been resealed with instant glue, and then, once the bombers were aboard the flights, connected with detonators made of AA batteries filled with the explosive HMTD and disposable cameras acting as triggers. Scotland Yard said at the time of the arrests that it had found plastic drink bottles and large quantities of hydrogen peroxide on premises used by the defendants, along with “martyrdom” videos taped by six defendants, in the manner common among Islamic suicide bombers.

Also found was a computer memory stick belonging to Mr. Ali, who described himself in court as the leader of the plot, that had files with highlighted schedules for seven flights to New York, Washington, Chicago, San Francisco, Montreal and Toronto between July and October 2006 aboard aircraft operated by American Airlines, United Airlines and Air Canada. The computer files included information on baggage rules, guidance as to what could be carried as hand luggage and information about Heathrow Airport in London, Scotland Yard said.

But the jury failed to reach verdicts against seven of the eight men on the most serious charge, that of conspiring “to murder persons unknown by the detonation of improvised explosive devices on board trans-Atlantic passenger aircraft.” By finding three of the men guilty of plotting murder, but not of an airliner bombing conspiracy, the jury appeared to have concluded that they had the means and the intention of detonating bombs, but that the prosecution had failed to prove conclusively that they had planned to attack airliners.