A key al Qaeda figure who had access to the surveillance data that led authorities to increase the terror alert level was among those arrested in raids in Britain on Tuesday, according to a senior U.S. national security official. He said the arrests were made on information obtained following the arrests of al Qaeda suspects in Pakistan.

The official described the arrest of Eisa al-Hindi, an al Qaeda leader in Britain, as the initial unraveling of a network of al Qaeda members believed to have been actively planning attacks in the United States and elsewhere.

Al-Hindi, according to the official, had access to the detailed surveillance of the five financial institutions in Washington, New Jersey and New York that was stored in the computer of a suspect apprehended earlier in Pakistan. Other al Qaeda operatives outside Britain also had access to the information, the official said.

The official also said investigators believe there may be a link between al-Hindi and other recent intelligence that has raised serious concerns among counterterrorism officials in Washington.

Al-Hindi was among 13 men, ages 19 to 32, arrested in raids late Tuesday in London, the nearby towns of Watford and Luton, and Blackburn in northwestern England. One man was freed Wednesday without charge. The others were being questioned at a London police station "on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism," police said, according to the Associated Press.

The arrests in Pakistan that led investigators to al-Hindi began in June with the apprehension by Pakistani paramilitary forces of Mussad Aruchi, an al Qaeda operative. The operation was supervised by the CIA, officials said. Aruchi, described as a nephew of Khalid Sheik Mohammed, the architect of the attacks of Sept. 11, 2001, told interrogators he "was sure that al Qaeda would hit New York or Washington pretty soon." He is also a cousin of Ramzi Ahmed Yousef, who was convicted of planning and carrying out an attack on the World Trade Center in 1993.

Aruchi's capture led to the arrest of Muhammad Naeem Noor Khan, a Pakistani, in the city of Lahore on July 13, and to the apprehension last week of Ahmed Khalfan Ghailani, a Tanzanian wanted in connection with the 1998 bombings of the U.S. embassies in Kenya and Tanzania.

Bush administration officials said the terror alert for financial sectors in Washington, New York and Newark was based in part on the contents of a laptop computer, disks and other materials seized during an arrest of an al Qaeda fugitive in Pakistan in late July showing that al Qaeda operatives had conducted detailed surveillance of the five buildings. U.S. officials did not make clear until Tuesday that the surveillance was conducted three to four years ago and that authorities were not sure whether it had continued.

U.S. intelligence officials have said that separate information received last week added to a sense of alarm within the administration and that intelligence on threats associated with the election period remained serious. One official described the information yesterday as relating to "a current threat and contemporaneous."

"The al Qaeda operative and the computer files, that's a stream of intelligence there," the White House press secretary, Scott McClellan, said yesterday. "But there is another new stream of intelligence that came to our attention on Friday, as well. . . . I can't go further into it because it could compromise some ongoing operations at this point."

A senior law enforcement official said yesterday that the intelligence comes from an al Qaeda source overseas who has said the terror group was planning an attack involving financial targets in New York before the November elections. The source did not identify a particular building, the official said.

The arrests in Britain added to jitters about a possible attack there and fueled suspicions among Muslims about being unfairly targeted.

A new parliamentary report supported those suspicions, saying there was "mounting evidence" that anti-terrorist legislation was being used disproportionately against Muslims, the Associated Press reported.

British police gave few details of the allegations against the men arrested Tuesday, saying only that the raids were part of a continuing "intelligence-led" investigation. British forensic officers searched several homes on Wednesday.

Police released no details of the nationalities or religious affiliations of those arrested, but news reports suggested that at least some of the suspects are Muslims from South Asia.

The arrests brought complaints from the Muslim community of unfair treatment, the Associated Press reported.

Of the 609 people -- most of them Muslim -- arrested under British anti-terrorism laws between Sept. 11, 2001, and the end of June, only 97 were charged with a terrorism-related offense, according to government figures. Half were released without charge.

Special correspondent Kamran Khan in Karachi, Pakistan, and staff writer Dan Eggen contributed to this report.