Federal investigators have detained a man they believe was a roommate last summer of at least two of the September 11th hijackers.
Mr Rasmi Al-Shannaq, a Jordanian citizen, was taken into custody for overstaying his visa, said an official.
The State Department's diplomatic security service and the FBI raided a house in Baltimore looking for Mr Al-Shannaq who was questioned about what he knew of the hijackers and any possible plans they might have discussed.
The men he lived with in the Washington suburbs - Mr Hani Hanjour and Mr Nawaq Al-Hazmi - are suspected of hijacking American Airlines Flight 77 just before it was crashed into the Pentagon.
The FBI is still searching for other men believed to have had close contact or lived with the hijackers, the official said.
He added the government's interest in Mr Al-Shannaq may never lead to charges, but investigators want to know if he might have heard something that would lead to other possible terrorist plans.
Earlier this month, the UD justice department said 73 people are being held in federal custody on criminal charges or violations related to the government's investigation of September 11th.
In addition, 74 others are being held on immigration-related charges and are in the custody of the Immigration and Naturalisation Service. Some of those held for immigration violations do not have lawyers.
PA