Call it what you will, but the government's use of military tribunals and its detention of hundreds of people are necessary tools to combat "sleeper cells" of terrorists quietly waiting to strike, a Justice Department lawyer testified Wednesday.

Assistant Attorney General Mike Chertoff said the military tribunals will be critical in catching off-guard Al Qaeda terrorists who have extensive training in how to take advantage of the cracks in the U.S. legal system.

"Woe unto us if we don't learn the lessons of what they're teaching," Chertoff warned the Senate Judiciary Committee.

He said an Al Qaeda manual found in the rubble of the 1998 embassy bombings in Africa had lesson plans on the ins and outs of the American legal system, and taught the art of hiding messages during detention visits and taking advantage of public sentiment during an indictment and trial period.

But Sen. Russ Feingold, D-Wis., representing a number of senators critical of the president's call for military tribunals, said he is concerned that "the very foundations of our systems are threatened."

"One of the few advantages I can see in all these changes being directed by the executive without adequate consultation is that it may make the terrorists' handbooks about how our system works obsolete," Feingold said.

Chertoff defended the president's decision, saying that Americans can't just keep relying on the good fortune that enabled the government to avert a millennium celebration terrorist attack on Los Angeles in December 1999 with the border arrest of Ahmed Ressam.

"We could continue this war and hope we get lucky as we did in the Ressam case," Chertoff said. "Or we can pursue a comprehensive and systematic investigative approach that aggressively uses every available, legally permissible investigative technique to identify, disrupt and, if possible, incarcerate and deport sleepers."

He also justified the government's controversial monitoring of jailhouse conversations between lawyers and suspects.

"Are we being aggressive and hard-nosed? You bet," Chertoff said. "In the aftermath of Sept. 11, how could we not be?"

Currently, the government is monitoring 16 prisoners — 12 of them in jail on terrorist charges unrelated to Sept. 11 and four on espionage charges.

Chertoff's comments came one day after his boss, Attorney General John Ashcroft, gave the most thorough public accounting of terrorism arrests so far, naming for the first time nearly all of the 104 people who have been charged with federal crimes.

Ashcroft declined, however, to identify the hundreds of people being held on immigration violations, suggesting some were members of Usama bin Laden's terror network.

"I am not interested in providing, when we are at war, a list to Usama bin Laden and the Al Qaeda network of the people we have detained that would make any easier their effort to kill Americans," Ashcroft said.

In the hearing, Chertoff also answered charges that the Justice Department was participating in ethnic profiling by requesting Arab Americans to come in to the FBI for interviews. He said the government was merely capitalizing on its knowledge of bin Laden's pattern of using individuals from certain countries traveling with specific visa classes.

The Department of Defense has not yet developed its rules for military courts, but the president's order requires minimum standards be afforded to defendants, including the right to an attorney, a full and fair trial of the charges and the determination whether a trial by jury is appropriate.

Several senators complained, however, that not only were the tribunals an opportunity to upend the justice system, but that Congress was not even consulted on the decision, even though it had just passed a law giving the Justice Department wide new powers to investigate and prosecute terrorists.

"Wouldn't it have made more sense if we're giving you all this extra authority anyway ... to come here and say 'look why don't you put in another section ... giving us specific authorization for the president as commander-in-chief to set up military commissions' thus removing the debate now going on in this country?" asked Senate Judiciary Committee Chairman Patrick Leahy, D-Vt.

The Associated Press contributed to this report.