The Bush administration is running into resistance from civil rights defenders in Congress as it pushes for a series of anti-terrorism laws. Tuesday Attorney General John Ashcroft lobbied his former Senate colleagues for tougher anti-terrorism measures.

The attorney general offered few details about the probe of the attacks on New York and Washington, but cautioned that further incidents are possible. The government is allowing small crop-dusting planes to fly again after grounding them amid fears they might be used in a chemical or biological strike. However, Mr. Ashcroft says the terrorists may be using other methods.

"Today I can report to you that our investigation has uncovered several individuals, including individuals who may have links to the hijackers, who fraudulently have obtained or attempted to obtain hazardous material transportation licenses," he said.

The administration and Congress are trying to come to terms on a package of new anti-terrorist measures. The Justice Department wants broader surveillance power over telephones and computers. The proposed legislation would also toughen penalties for those who harbor terrorists.

But the plan has drawn opposition from lawmakers who complain the administration is moving too far and too quickly. Liberal Democrats and conservative Republicans alike have found things to criticize.

One of the most controversial steps would allow immigrants who are suspected of terrorist ties to be detained indefinitely without trial. The president of the American Civil Liberties Union, Nadine Strossen, says the group is opposed to this idea.

"In order to incarcerate somebody, basically imprison them, completely prevent their freedom of movement, the kind of punishment that is meted out only after a full fledged trial, can not be done until somebody has actually been convicted of a crime," she said.

Mr. Ashcroft defends the various proposals, but concedes they might not have prevented the deadly airborne assault two weeks ago.

"We know that these provisions would strengthen our ability to know and to intercept and to monitor, and prevent, to thwart and disrupt," he said. "But there is no way for me to guarantee that even with all of these items, we would have known about this particular plan and effort."

Authorities have arrested or detained more than 350 people since the attacks and are searching for hundreds more.