WASHINGTON, Aug. 1 – The unannounced capture of a figure from Al Qaeda in Pakistan several weeks ago led the Central Intelligence Agency to the rich lode of information that prompted the terror alert on Sunday, according to senior American officials.
The figure, Muhammad Naeem Noor Khan, was described by a Pakistani intelligence official as a 25-year-old computer engineer, arrested July 13, who had used and helped to operate a secret Qaeda communications system where information was transferred via coded messages.
A senior United States official would not confirm or deny that Mr. Khan had been the Qaeda figure whose capture led to the information. But the official said “documentary evidence” found after the capture had demonstrated in extraordinary detail that Qaeda members had for years conducted sophisticated and extensive reconnaissance of the financial institutions cited in the warnings on Sunday.
One senior American intelligence official said the information was more detailed and precise than any he had seen during his 24-year career in intelligence work. A second senior American official said it had provided a new window into the methods, content and distribution of Qaeda communications.
“This, for us, is a potential treasure trove,” said a third senior American official, an intelligence expert, at a briefing for reporters on Sunday afternoon.
The documentary evidence, whose contents were reported urgently to Washington on Friday afternoon, immediately elevated the significance of other intelligence information gathered in recent weeks that had already been regarded as highly troubling, senior American intelligence officials said. Much of that information had come from Qaeda detainees in Afghanistan and Saudi Arabia as well as Pakistan, and some had also pointed to a possible attack on financial institutions, senior American intelligence officials said.
The American officials said the new evidence had been obtained only after the capture of the Qaeda figure. Among other things, they said, it demonstrated that Qaeda plotters had begun casing the buildings in New York, Newark and Washington even before the attacks of Sept. 11, 2001.
Among the questions the plotters sought to answer, senior American intelligence officials said, were how best to gain access to the targeted buildings; how many people might be at the sites at different hours and on different days of the week; whether a hijacked oil tanker truck could serve as an effective weapon; and how large an explosive device might be required to bring the buildings down.
The American officials would say only that the Qaeda figure whose capture had led to the discovery of the documentary evidence had been captured with the help of the C.I.A. Though Pakistan announced the arrest last week of a Qaeda member, Ahmed Khalfan Ghailani, a Tanzanian wanted in connection with the bombings of American embassies in East Africa in 1998, the American officials suggested that he had not been the source of the new threat information.
An account provided by a Pakistani intelligence official made clear that the crucial capture in recent weeks had been that of Mr. Khan, who is also known as Abu Talha. The intelligence official provided information describing Mr. Khan as having assisted in evaluating potential American and Western targets for terrorist attacks, and as being representative of a “new Al Qaeda.”
The Pakistani official described Mr. Khan as a fluent English speaker who had told investigators that he had visited the United States, Britain, Germany and other countries. Mr. Khan was one of thousands of Pakistani militants who trained in Afghanistan under the Taliban in the 1990’s, the Pakistani official said.
If indeed Mr. Khan was the man whose arrest led the C.I.A. to new evidence, his role as a kind of clearinghouse of Qaeda communications, as described by the Pakistani intelligence official, could have made him a vital source of information. Since his arrest, Mr. Khan has described an elaborate communications system that involves the use of high and low technology, the Pakistani official said.
The question of how much to rely on information obtained from captured foes has always weighed on the intelligence business. In recent weeks, even as they cited accounts from some captured Qaeda members as the basis for new concerns about terrorism, American intelligence officials have acknowledged that another captured Qaeda figure, Ibn al-Shaykh al-Libi, had recanted claims that Iraq had provided training in illicit weapons to Qaeda members.
Mr. Libi’s earlier claims had been the primary basis for assertions by President Bush and his top advisers that Iraq had provided training in “poisons and gases” to Qaeda members.
In explaining the decision to call a new terror alert, American officials would say only that the evidence obtained by the C.I.A. after the arrest of the Qaeda figure in Pakistan had provided a richer, more credible source of intelligence than could have been provided by any single individual. They declined to say whether the “documentary evidence” included physical documents or might also include electronic information stored on computers, like copies of e-mail communications.
The Qaeda communications system that Mr. Khan used and helped operate relied on Web sites and e-mail addresses in Turkey, Nigeria and the northwestern tribal areas of Pakistan, according to the information provided by a Pakistani intelligence official.
The official said Mr. Khan had told investigators that couriers carried handwritten messages or computer disks from senior Qaeda leaders hiding in isolated border areas to hard-line religious schools in Pakistan’s Northwest Frontier Province.
Other couriers then ferried them to Mr. Khan on the other side of the country in the eastern city of Lahore, and the computer expert then posted the messages in code on Web sites or relayed them electronically, the Pakistani official said.
Mr. Khan had told investigators that most of Al Qaeda’s communications were now done through the Internet, the official said. After a message was sent and read by the recipient, the entire communication and related files were deleted to maintain secrecy, he said. Mr. Khan had told investigators that e-mail addresses were generally not used more than a few times.
The young computer engineer, who received a bachelor’s degree from a university in Karachi, is the unemployed son of an employee of Pakistan’s state airline and a college botany professor, the official said. Heavily built and 6 feet 2 inches tall, he speaks English with a British accent, and was arrested carrying a fake Pakistani identification card.
The Pakistani official said Mr. Khan told investigators that he had received 25 days of training at a militant camp in Afghanistan in June 1998. By the time Mr. Khan had risen to his current position, the official said, Qaeda figures had arranged his marriage and were paying him $170 a month for rent for his house in Lahore and $90 for expenses.
Mr. Khan was in contact with the brother of the Indonesian Qaeda leader Hambali, who was studying in a religious school in Karachi, and who was deported in December 2003. Mr. Khan has told interrogators that his Qaeda handler was a Pakistani he knew as Adil or Imran, who assigned him tasks related to computer work, Web design and managing the handler’s messages. His correspondents included a Saudi-based Yemeni, Egyptian and Palestinian nationals and Arabs in unknown locations, and someone described as the “in-charge” in the city of Khost in eastern Afghanistan.
Asked about the whereabouts of Osama bin Laden and his deputy, Ayman al-Zawahiri, Mr. Khan has told interrogators that even the top Qaeda commanders do not know, the Pakistani intelligence official said.
Douglas Jehl reported from Washington for this article, and David Rohde from Karachi, Pakistan.