(CNN) -- A drop in so-called "chatter" among suspected terrorists is troubling some counterterrorism officials, who noticed a reduction in intercepted communications before the terror attacks of September 11, 2001, government sources said.
Diminished communication prompted the concern because the counterterror experts don't know why suspected terrorists would be talking less. But they noted that similar reductions have happened several other times during the past few years.
Pakistan authorities arrested a key al Qaeda suspect, Muhammed Naeem Noor Khan, in mid-July. Officials said chatter continued after that arrest, but has fallen off in recent days.
Khan's arrest was announced this week.
Within the past few days a series of arrests in Britain and Pakistan produced thousands of leads, particularly intelligence related to two men allegedly involved in helping al Qaeda operatives communicate with each other: Khan and Esa al-Hindi.
Sources said some of the intelligence information being pursued includes phone numbers and e-mail addresses that the FBI and other agencies are trying to track down to locate any al Qaeda operatives in the United States.
U.S. government officials describe al-Hindi as a "major player who moved operational information between key components of al Qaeda" in Britain, the United States and Pakistan.
Born of Indian parents, al-Hindi converted to Islam at 20 and fought in Kashmir, a disputed border region between India and Pakistan.
Now in his mid-30s, al-Hindi wrote a book called "The Army of Medina and Kashmir," in which he advocated attacking the economies of Western powers as the best way to get them to disengage from the Muslim world.
One source told CNN that law enforcement authorities have placed al-Hindi in three of the financial buildings that were cased: the New York Stock Exchange and the Citigroup building in New York City, and the Prudential Financial building in Newark, New Jersey.
Earlier this week, U.S. officials said images found on Khan's computer contained details about buildings in New York, New Jersey and Washington, D.C. (Full story)
After meeting with the FBI to review photographs contained in the surveillance reports, Prudential officials believe none of the photos of its headquarters in Newark were taken after 2001, spokesman Bob DeFillippo said Friday.
Company executives were able to verify that the photos weren't taken within the past three years because they did not contain surveillance cameras installed on the building's exterior after September 11, 2001, he said.
DeFillippo also said most of the photos appeared to have been taken from a book on the company's history that was published in 2001.
Al-Hindi was one of a dozen suspects arrested Tuesday in Britain whom authorities have accused of being part of a terror cell. The British authorities also said the United States has been particularly interested in al-Hindi for some time. (Full story)
Government sources told CNN that surveillance reports found recently in Pakistan contain details about the U.S. facilities written in perfect English. U.S. authorities said al-Hindi is known to speak perfect English.
Senior Pakistani intelligence officials said Khan spoke of a terror network in Britain and said he frequently relayed messages from Pakistan to an important al Qaeda operative.
British authorities said Heathrow Airport was one of several places in London that were uncovered by the Pakistani investigation, which turned up photos of potential targets.
