WASHINGTON -- Federal authorities are gathering evidence that suggests a small network of Islamic men helped fund and protect some of the 19 suicide attackers by providing cash, documents and possibly even safe houses, a high-ranking law enforcement official said Wednesday.

The official added that among the handful of individuals arrested so far, three are receiving particular scrutiny: a man arrested on immigration charges in Minnesota last month and two others pulled from a train in Texas on the day of the Sept. 11 terrorist attacks.

Authorities also are interested in the prior activities and associates of a physician arrested in San Antonio, Texas, who also is in federal custody.

Officials said they think that the two men on the train, Ayub Ali Khan and Mohammed Jaweed Azmath, were headed to San Antonio to meet with Dr. al-Badr al-Hazmi. The source also said that FBI and CIA intelligence officials were advised in August that as many as 200 Islamics with terrorist leanings were slipping into this country and planning "a major assault on the United States."

The advisory, passed on by the Mossad, Israel's intelligence agency, cautioned that it had picked up indications of a "large-scale target" in the United States and that Americans would be "very vulnerable," the official said.

How U.S. authorities reacted to the warning is not known, but the official said the advisory linked the information "back to Afghanistan and Osama bin Laden."

"There was a connection there," he said.

Attorney General John Ashcroft has said that authorities suspect that more airplanes were going to be hijacked, and that other co-conspirators -- possibly handlers and associates of the 19 suicide attackers -- remain at large.

Mindy Tucker, spokesperson for the Justice Department, said Wednesday that "we believe there are associates of the hijackers that have connections to the terrorist network that are present in the United States."

Other law enforcement authorities said such logistical support is typical within many Islamic terrorist cells.

Some participants help others slip unnoticed from city to city, and country to country, by providing them with fake or fraudulent passports, cash gained through bank- and credit-card fraud, and safe havens in their homes or in apartments rented under aliases, the authorities said.

On the morning that the Trade Center towers were destroyed, Habib Zacarias Moussaoui was in a Minnesota jail on an Immigration and Naturalization Service violation. According to one jailer, Moussaoui saw the disaster on television, and then stood and cheered.

Moussaoui's parents were born in Morocco, and he is a French citizen, born in the southern town of St. Jean de Luz in May 1968, according to an official at the French embassy in London. It was reported earlier that he was a French Algerian.

According to press reports, Moussaoui earned vocational degrees in automotive mechanics. On his university application, he expressed particular interest on his university application in learning business English so he could travel and "work in an international business."

French officials confirmed that Moussaoui was on a special immigration watch list because of his suspected ties to Islamic terrorists and because he had made several trips to Afghanistan.

Moussaoui spent at least three years in Britain in the late 1990s, according to French officials. He came to the French embassy in London in September 2000, and had his French passport extended. At the time, he described himself as unemployed and said he had lived at several addresses in the suburbs of London.

By this year, however, he was able to afford to journey to the United States and begin flying lessons. He was arrested Aug. 17 after the staff at a flight school grew concerned about his offer of thousands of dollars in cash for instruction in how to fly jumbo jets and his lack of interest in learning to take off or land jets.

The two men authorities removed from the train in Fort Worth, Texas -- Khan, 51, and Azmath, 47 -- also had a large sum of money with them -- $20,000 in cash, the official said -- as well as box cutters similar to those allegedly used by the hijackers on at least one of the commandeered planes.

The men had boarded a flight in Newark, N.J., that was bound for San Antonio on the morning of the attacks. But the flight was diverted to St. Louis after the World Trade Center was hit, and Khan and Azmath then took an Amtrak train to Texas.

The train was stopped in Fort Worth on a routine check for drugs, and Khan and Azmath were detained because of the materials and cash they were carrying. The train's final destination was San Antonio.

In San Antonio, federal authorities are continuing to scrutinize al-Hazmi, a 34-year-old Saudi physician. He was arrested after a search of his home and the medical school where he was finishing a five-year residency in radiology.

Officials of the University of Texas Health Science Center, where al-Hazmi studied, said he did not show up for his rounds on the day after the attacks, but they added that that was not noteworthy since he may have been studying for an important test.

Also Wednesday, owners of fitness clubs in Florida and Maryland said several of the suicide hijackers had worked out in their gyms.

"They may have been told to go get as strong as they could get in case of body conflict or a fight," said Jim Woolard, who owns eight World Gyms in Palm Beach and Broward counties.

Ziad Jarrah, a hijacker on the plane that crashed in Pennsylvania, made no secret of his aim: to learn how to fight.

On May 6, he signed up for a two-month membership, later extended to four, at the U.S. 1 Fitness Center in Dania Beach, north of Miami.

"He told me that he was from Germany, that he was visiting," said Roxanne Caputo, in charge of sales. "He would come in every day, sometimes twice."

Jarrah took classes in various combat techniques, including full-contact boxing, kick-boxing and the Brazilian martial art of Kopthaikido, Caputo said. He made two cash payments of $500 each to owner Burt Rodriguez to get some private one-on-one instruction.

Rodriguez recalled that his former pupil was soft-spoken, physically fit and a diligent learner, but that he lacked the "spark" of a born combatant.

"I've seen a lot of guys with the gloves on, and he was the kind who just wanted to survive," Rodriguez said.

During 17 lessons with the Cuban-born instructor (Jarrah missed the three final sessions he had paid for), he was taught how to grapple, defend himself in close quarters and protect himself from somebody wielding a knife or stick.

A hijacker could have used those same skills to overwhelm a flight crew or fight with airline passengers, his former teacher acknowledged with regret.

"To defend yourself, you obviously learn how to attack, which is the other side of the equation," Rodriguez said. "If he wasn't one of the pilots, he would have done quite well in thwarting the passengers from attacking."

In the summer, five hijackers on the two planes that crashed into the World Trade Center -- Mohamed Atta, Marwan Al-Shehhi, Wail M. Alshehri, Waleed M. Alshehri and Satam Al Suqami -- purchased one-month memberships at Woolard's gyms. Atta and al-Shehhi paid to work out at the Delray Beach gym; the others in Boynton Beach.

"They may have been doing it for social reasons, or to get strong for the upcoming battles," Woolard said of the men.

Five men identified as the hijackers of the plane that slammed into the Pentagon also worked out in the week before the attacks. While living in a rundown motel on the outskirts of suburban Laurel, Md., they showed up in various groupings every day from Sept. 2-6 at a nearby Gold's Gym. Three of them -- Khalid al-Midhar, Majed Moqed and Hani Hanjour -- paid $30 in cash for a week-long membership, while two others -- Salem and Nawaq Alhamzi-- paid $10 for each visit.

They spent their time training with weights and resistance machines, said Gene LaMotta, president and CEO of Gold's Gym. The fitness counselor said the men had "wads" of cash. And when the counselor asked if they could translate their Arabic names into English, Hanjour said his first name meant "warrior."