Government agents have recently uncovered numerous calls from difficult-to-track prepaid cell phones, Internet-based phone service, prepaid phone cards and public pay phones in the United States to known Al Qaeda locations overseas, federal officials said.

The calls are one piece of a growing body of evidence pointing to the presence of suspected members of terrorist sleeper cells operating on U.S. soil, and a growing sophistication on their part to keep their communications secret, the officials said.

The officials, who spoke only on condition of anonymity, said the effort to follow the phone call trail has involved numerous federal agencies and is the result of improved post-Sept. 11 coordination between the traditional law enforcement of the FBI and the intelligence gathering of the National Security Agency, America's premier overseas electronic intercept agency.

"Things have really improved, and that gives us the ability to better track terrorists both in the United States and abroad, and prevent things before they happen," one senior law enforcement official told The Associated Press.

The officials said the process works like this: U.S. intelligence learns of a communication to known Al Qaeda locations overseas and then alerts the FBI and other law enforcement agencies, who try to track down the source and origin of the U.S. callers.

Authorities said the calls point to the clear presence of one or more sleeper cells in the United States and attempts by Al Qaeda sympathizers in America to make their calls difficult to trace, using tactics invented by U.S. criminals in the 1990s.

With Friday's arrest of five American men of Yemeni descent in a Buffalo, N.Y. suburb, Deputy Attorney General Larry Thompson said that U.S. law enforcement "has identified, investigated and disrupted an Al Qaeda-trained terrorist cell on American soil."

In other recent steps to disrupt suspected domestic terrorist activities, the indictment of several men in Detroit cited the possible presence in the Midwest of a "combat squad" of terrorists. Also, the government in the past few weeks charged a man with trying to help Al Qaeda set up a terrorist training camp in Oregon.

Investigators have found several instances in which suspected Al Qaeda calls from the United States to overseas numbers were made on prepaid cell phones, prepaid telephone credit cards or public phone booths. One official said there also has been some instances of suspect calls made through Internet-based phone services.

"All this shows is that they are about as sophisticated as mafia guys from the 1990s," said Stewart Baker, a Washington lawyer who formerly served as the NSA's general counsel.

Baker said more significantly, the operation of Al Qaeda on U.S. soil, including the Sept. 11 hijackers, suggests a change in terrorists' method of operation. Instead of entering the country just before they perpetrate attacks, they now stay on U.S. soil for months and even talk on U.S. communications system as they wait to carry out orders.

"It is clear that they feel comfortable operating here, and that is a new problem for us," Baker said, adding that the solution is to rapidly improve the domestic intelligence gathering capabilities of U.S. law enforcement.

"Now we have to begin looking very closely at people who haven't committed any crimes yet because of the concern that the first one they'll commit is going to be a doozy," he said.

The feared use of prepaid cell phones and cards by criminals and terrorists is not new. Attorney General John Ashcroft cited it as one reason why Congress needed to pass the Patriot Act and expand the FBI's surveillance powers so agents could track people who changed cell phones to elude FBI surveillance.

Usama bin Laden used prepaid minutes on a satellite phone in Afghanistan during the 1990s, thinking it would disguise his communications, according to past court testimony. Officials have said bin Laden stopped using the phone when it became known it was being monitored by U.S. intelligence.

Oklahoma City bombing convicts Timothy McVeigh and Terry Nichols used a prepaid telephone card to make calls from public pay phones and other places in the two years before that 1995 attack was carried out. The card was put in a fake name, Daryl Bridges.