A worldwide manhunt is on for a Yemeni man who was supposed to be the 20th hijacker on Sept. 11.

Ramzi Omar, also known as Ramsi Binalshibh, is believed to have intended to be part of the terror team that hijacked United Airlines Flight 93, which crashed in southwestern Pennsylvania.

Binalshibh was never able to enter the United States, despite three attempts by Mohamed Atta, the suspected ringleader of the 19 hijackers, to get him into the country before Sept. 11, FBI Director Robert Mueller told federal prosecutors Wednesday at a briefing.

"We believe he was the 20th hijacker," Mueller said. The FBI director said the teams that commandeered and crashed four commercial airliners had five hijackers each except the United flight that crashed in a Pennsylvania field while on a flight path to Washington.

Mueller's assertion about Binalshibh marks a change from earlier suggestions by federal authorities that a man arrested in Minnesota, Zacarias Moussaoui, may have been the 20th hijacker.

Justice Department and FBI officials refused comment on Mueller's remarks.

A month ago, Vice President Dick Cheney said Moussaoui, who was taken into custody the month before the hijackings, may have been intended as part of the terrorist crew that commandeered Flight 93.

Mueller told prosecutors Wednesday there was no information on the computer seized from Moussaoui that links him to the Sept. 11 attacks. That prompted officials to consider other suspects as the 20th hijacker, officials said.

At a security conference in Germany on Wednesday, FBI official Michael Rolince said that "as an investigator I'm convinced there were supposed to be five people on this plane. ... Whoever that fifth person was is probably still alive.

"Clearly we are looking into the pool of people who crossed paths with the hijackers" to find the 20th hijacker, said Rolince, FBI section leader for international terrorism.

U.S. officials, speaking on condition of anonymity, said information recently obtained from Usama bin Laden operatives now in custody has helped provide a clearer picture of the hijacking plot and plans for follow-up attacks.

German authorities have issued international arrest warrants for three suspected accomplices of the hijackers: Binalshibh; Said Bahaji, a German national; and Zakariya Essabar of Morocco. All three left Hamburg shortly before the Sept. 11 attacks.

Attorney General John Ashcroft has said the three had extensive connections to Atta and Marwan Al-Shehhi, the suspected pilots of the hijacked planes that crashed into the World Trade Center in New York City, and Ziad Jarrah, suspected of flying the plane that crashed in Pennsylvania.

In the closed-door meeting with prosecutors, Mueller offered details about Moussaoui, saying that when the FBI searched his computer, it contained information about "dispersal of chemicals" as well as about crop-duster planes.

The discovery prompted the Bush administration to temporarily ground crop-dusters as a precaution against a possible biochemical terrorist attack.

Mueller also said the news media incorrectly have reported Moussaoui's interest in learning to handle airplanes at a time when he was taking flight training.

Mueller said Moussaoui wanted to learn how to take off and land, but not fly.

"Newspapers have it the other way," Mueller noted.

Moussaoui was detained Aug. 17 on immigration charges after officials at a flight school where he sought training grew suspicious and called authorities. He is being held as a material witness — someone with possibly important information — in the probe of the terrorist attacks.

The Associated Press contributed to this report.