Federal investigators are saying little about the people they have rounded up for questioning in the U.S. in the wake of the Sept. 11 terror attacks, but a handful of the suspects believed to be key to the investigation are in custody as material witnesses or for immigration violations.

Only three people have been formally charged, men in Detroit allegedly found in possession of falsified travel documents. The exact number of people brought in for questioning is not known, but is believed to be in the hundreds worldwide.

Nabil Al-Marabh, 34

Status: In custody in New York

Al-Marabh, who listed addresss in Massachussetts, Illinois and Canada since 1994, was arrested Sept. 19 at a Burbank, Ill., convenience store. Al-Marabh once worked for Boston Cab Co., where an associate of suspected terror chief Osama bin Laden also worked.

Nabil Al-Marabh

The FBI linked Al-Marabh to Raed Hijazi, now jailed in Jordan on charges that he planned to blow up a hotel filled with Americans and Israelis on New Year's Day 2000.

Al-Marabh holds a commercial driver's license and is certified to transport hazardous materials. Canadian media reported that a man with the same name and age lived in Toronto in the mid-1990s and was collecting welfare while waiting word on his refugee application; it was denied. Authorities will not say why they were seeking Al-Marabh.

Ahmed Hannan, 33, Farouk Ali-Haimoud, 21, Karim Koubriti, 23

Status: In custody in Detroit

Ahmed Hannan, Farouk Ali-Hamoud and Karim Koubriti were arrested Sept. 18 by the FBI inside a Detroit apartment where agents expected to find Nabil Al-Marabh.

Ahmed Hannan

U.S. Magistrate Judge Virginia Morgan ordered Farouk Ali-Haimoud, 21, Ahmed Hannan, 33, and Karim Koubriti, 23, held without bond on charges of possession of false documents — passports, Social Security Cards and visas.

The FBI also allegedly found the three men with documents in Arabic that may be related to a foiled attack on a U.S. airbase in Turkey.

Farouk Ali-Haimoud

Agents found identification badges for LSG Sky Chefs, an airline caterer at Detroit Metropolitan Airport.

Karim Koubriti

Two men worked for the company briefly this summer washing dishes, but reportedly did not have access to airplanes or secure areas. The men, who told authorities they were from Morrocco and Algeria, are being held without bond.

Zacarias Moussaoui, 33

Status: In custody in New York

Zacarias Moussaoui, a French Algerian arrested Aug. 17 in Minnesota, is believed to be one of a handful of "material witnesses" being held in New York for possible presentation to a grand jury convened in White Plains.

Zacarias Moussaoui

Moussaoui attraction attention to himself before the attacks when he allegedly sought training on a jumbo jet simulator at a Minnesota flight school. He reportedly told an instructor he just wanted to learn steering, not landing. Moussaoui last spring took aviation courses in Norman, Okla., but he failed to obtain a pilot's license.

Ayub Ali-Khan, 51, Mohammed Jaweed Azmath, 47

Status: In custody in New York

Ali-Khan and Azmath are believed to be the first material witnesses to be arrested in the global manhunt, detained just hours after the attack after being pulled form a San Antonio-bound Amtrak train in Forth Worth, Texas.

Ayub Ali-Khan

Ali-Khan and Azmath earlier had been headed to San Antonio aboard a plane that left Newark about the time of the attacks; the plane, however, was grounded in St. Louis because of the attacks.

Mohammed Jaweed Azmath

Sources told CNN and the Associated Press that the men had large amounts of cash, hair dye and box cutters in their possession. CNN reported that both were cooperating with investigators. Indian police said the men may actually be Gul Mohammed Shah, 32, and Mohammed Jaweed Azmat, also 32.

Al-Bader Al-Hazmi, 34

Status: In custody in New York (no photo available)

Investigators believe Al-Bader Al-Hazmi, a radiologist working in San Antonio, Texas, might have provided technical or financial support to at least one of the teams of terrorists who hijacked four planes. Al-Hazmi, a Saudi national, is being held by federal agents in New York as a material witness but not has been charged.

A computer and records were seized from Al-Hamzi's home and a library at the University of Texas Health Science Center, where has in residency. Al-Hamzi was one of at least four people on the FBI's watch list booked on a United Airlines flight that was scheduled to travel Sept. 22 from San Antonio to San Diego, with a stop in Denver. His last name is similar to that of suspected hijackers Nawaq and Salem Alhamzi, believed to have died aboard the American Airlines jet that crashed into the Pentagon.

News organizations around the country have been reporting that a number of people were detained by FBI agents following the extensive e-mail, telephone and cellphone trail left by the hijackers.

Florida papers reported that an unidentified 43-year-old travel agent from Orlando was taken to New York for questioning. Authorities have yet to disclose what become a man arrested at New York's John F. Kennedy International Airport found in possession of his brother's pilot license.