BAGHDAD – American troops seized the computer of Abu Musab al-Zarqawi, Al Qaeda’s leader in Iraq, when they came tantalizingly close to capturing the terrorist after chasing a truck in which he was traveling, it was disclosed yesterday.

A senior military official described finding the laptop as “a seminal event” that would help to run to ground the Jordanian-born militant and unravel his network, responsible for many of Iraq’s worst outrages, including the beheading of the British contractor, Ken Bigley.

Details of the computer’s discovery showed how close America was to detaining Mr. Zarqawi – who, with a $25 million bounty on his head, is their no. 1 target in Iraq – and the extensive precautions he takes to avoid detection.

The covert American military unit charged with finding him, known as Task Force 626, was alerted by a source inside Mr. Zarqawi’s network on February 20 that he was to attend a meeting at Ramadi, the city west of Fallujah that he is believed to use as his operational base.

Mobile vehicle checkpoints were set up around the city, and predator drones scrambled to track all movements.

Just before the meeting was scheduled, a car on its way from Fallujah was pulled over as it approached a checkpoint. This is now believed, the American officer said, to have been a support car sent ahead to “check the waters.”

A pickup truck half a mile behind the car was seen turning around and speeding off in the opposite direction. Anerican teams gave chase, but when the truck was stopped several miles away, there was no sign of Mr. Zarqawi.

Instead, there was evidence of how close they had been to inflicting a massive blow to al Qaeda. The computer had several plug-in hard drives that identified it as Mr. Zarqawi’s, as well as tactical information and photographs of the man. It emerged that his currency of choice is the euro, with 80,000 euro notes discovered in the vehicle.

The two men inside, Mr. Zarqawi’s driver and bodyguard, were taken into custody.

The Americans said they had since learned that Mr. Zarqawi jumped out of the vehicle when it passed beneath an overpass to avoid detection from the air and hid there before running to a nearby safe house. That house was raided and the owner arrested.

Lieutenant General John Vines, responsible for daily military operations in Iraq, recently said he believed American operations had damaged Mr. Zarqawi’s network and hampered its ability to operate in the country.