The man's name was not released. The newspaper said it learned of the arrest from law enforcement official who spoke on condition of anonymity.
FBI spokesman Paul McCabe declined to comment on the report. U.S. Attorney Tom Heffelfinger didn't return a call from The Associated Press.
The Star Tribune source said the detainee has confirmed some of investigators' suspicions about Moussaoui, who was arrested while learning to fly a Boeing 747 jet at a Minnesota flight school two years ago. The official told the newspaper that the jailed man has described Moussaoui's activities at an al Qaeda training camp in Afghanistan several years ago.
Members of the FBI's local Joint Terrorism Task Force made the arrest Tuesday after a lengthy investigation, the official said. But the detainee's identity was kept off the public roster of inmates at the Hennepin County jail.
Roseann Campagnoli, a spokeswoman for the Hennepin County Sheriff's Office, referred calls Wednesday to the U.S. attorney's office.
The Star Tribune's source said the suspect was served with an arrest warrant Tuesday and brought before U.S. Magistrate Judge Earl Cudd, but the proceedings were sealed.
Moussaoui, 35, was indicted on Dec. 11, 2001. He is charged with a broad conspiracy that includes — but is not limited to — the Sept. 11 attacks, in which he denies any involvement. His is the only U.S. criminal case spawned by the suicide hijackings.