The FBI has detained 49 people for questioning and is searching worldwide for as many as 200 others following the terrorist attacks on America.
FBI Director Mr Robert Mueller confirmed that several secret material witness warrants have been issued for the arrest of people who are believed to have information about the atrocities.
He declined to provide estimates or names, saying courts have sealed the information. The US Justice Department previously confirmed two people were arrested on such warrants.
Mr Mueller said intelligence agencies did not have any advance indications about the suicide hijackings that struck the World Trade Center towers and the Pentagon.
Attorney General Mr John Ashcroft asked the US Congress to pass sweeping new anti-terrorism measures by the week's end to help the inquiry. He also assigned numerous federal agents to provide security aboard commercial airliners.
"Associates of the hijackers that have ties to terrorist organisations may be a continuing presence in the United States," Mr Ashcroft said.
But no one has been charged directly with crimes related to the September 11th attacks. Some of those detained have been charged with immigration violations and have requested lawyers, Justice Department officials said.
One of the men suspected of crashing a jetliner into the Pentagon flew small planes over the Washington area at least three times in the past six weeks while taking flight training in suburban Maryland.