ASHINGTON, Sept. 15 ?The Congressional leaders who oversee the nation's intelligence system have concluded that America's spy agencies should be allowed to combat terrorism with more aggressive tactics, including the hiring of unsavory foreign agents.

The attacks on the World Trade Center and the Pentagon have also revived discussion of reversing the United States' 25-year ban on using covert agents to assassinate foreigners. A consensus has not been reached on that point.

But after the attacks, the chairman and vice chairman of the Senate intelligence committee, the chairman of the House intelligence committee and two former directors of central intelligence said the attacks justified easing some restrictions on the behavior of spy agencies. Some of those leaders also said the terrorist assault represented a colossal failure of American intelligence.

"We have got to be a hell of a lot more aggressive," said Senator Richard C. Shelby, Republican of Alabama and vice chairman of the Senate intelligence committee.

R. James Woolsey, the former director of central intelligence, said that "Washington has absolutely undergone a sea change in thinking this week."

Those comments reflect a turning point in the attitude of political leaders toward the need for sharp limits on the extent and nature of covert operations and perhaps for allowing American agents to carry out the kinds of actions that have long been prohibited as too ruthless or morally questionable.

They also reflect a strong public sentiment for a powerful, and prolonged, American assault on the terrorist organizations responsible for the deaths of thousands of people in New York and Washington, and others like them. A New York Times/ CBS News poll conducted late last week showed that 65 percent of those questioned say American agents should be allowed to seek out and assassinate people in foreign countries who commit terrorist acts against Americans.

For the moment, the C.I.A. is not pressing Congress or the White House to change its rules. Administration officials said they understood that for many Americans the ban on assassinations was a significant symbol of the nation's role as a standard-bearer of ethical conduct. Under current law, President Bush would have to authorize personally any such change in the existing executive order governing intelligence operations.

But the public discussion among influential members of Congress about freeing the C.I.A. from restrictions on the recruitment of criminals and known abusers of human rights as informants and about outlaw assassinations stems from a growing debate over the causes of what many in Washington are now calling the nation's biggest intelligence lapse since the surprise attack on Pearl Harbor in 1941.

In the 25 years since Congressional hearings disclosed the agency's role in assassinations and dirty tricks overseas, the government has imposed increasingly tighter rules and Congressional oversight of the conduct of America's spies. C.I.A. officers, for example, are not permitted to foster a plot that has the explicit goal of killing a terrorist leader.

But Congressional leaders said the C.I.A. should be put on a war footing and given the freedom not only to penetrate but also to destroy tightly knit terrorist organizations.

"Not everybody is playing by Marquess of Queensberry rules," said Representative J. Porter Goss, a Florida Republican who is chairman of the House Permanent Select Committee on Intelligence when asked if he would support an end to the ban on assassinations of foreign leaders, first imposed by President Gerald R. Ford in 1976.

Senator Bob Graham, a Florida Democrat who is chairman of the Senate Select Committee on Intelligence, is now willing to end limits on the C.I.A.'s recruitment of agents ?spies ?who have committed human rights violations, his spokesman said on Friday.

But one influential lawmaker warned that proposals to unleash intelligence agencies should be carefully considered.

Representative Tom DeLay of Texas, the majority whip, said that while the C.I.A. should beef up its human intelligence gathering ability, officials should not move rashly to lift the ban on assassinations.

The nation is understandably "a little panicky," Mr. DeLay said. But he added, "I think we need to be very clearheaded, very deliberative, about where we're headed."

Tighter restrictions on whom the C.I.A. can recruit as spies were imposed in the mid-1990's after disclosures about the agency having had ties to a Guatemalan Army officer implicated in the killing of an American and the husband of another American. Although it is unclear whether the guidelines have ever really undercut the C.I.A.'s operations against terrorist organizations, the agency's officers have complained in the past that the rules were symbolic of a broader caution that took hold at the agency in the 1990's, when managers rejected high-risk operations for fear they would fail or lead to political scandal.

Senator Graham's spokesman, Paul Anderson, said, "The senator said something today that I hadn't heard him say before, and that is that we are not going to find the kinds of spies we need in monasteries."

Senator Graham said immediately after Tuesday's attacks that he was also willing to reassess the assassination ban. Mr. Anderson, the spokesman, said Mr. Graham had since modified his stance, but only because he had been told by experts that the United States could get around the ban if it chose to do so, even with the current legal strictures.

It remains to be seen whether Congressional leaders will continue to recommend that the C.I.A. be liberated from such restrictions once the heated passions in the aftermath of Tuesday's attacks begin to cool. Previous terrorist attacks like the 1998 bombings of two American Embassies in Africa have been followed by calls to loosen the limits on the C.I.A.'s operations. Last year, an independent commission on terrorism recommended that the C.I.A. lift its guidelines on the use of agents who had committed human rights violations. The proposals were ultimately not embraced by the government, and C.I.A officials argued at the time that it was not necessary to lift these restrictions because they had not hampered operations.

But the new willingness among lawmakers to allow the C.I.A. greater latitude underscores the depth of feeling in Washington about the need to address the intelligence failings exposed on Tuesday.

Former President George Bush, who served as C.I.A. director under President Ford, spoke publicly this week about the need to "free up the intelligence system from some of its constraints."

The militant attitude in Congress comes just weeks after some American leaders were sharply critical of Israel's use of assassinations of Palestinian leaders in response to a series of suicide bombings against Israeli targets. But after Tuesday some current and former officials said that American security services might need to adopt some elements of the Israeli approach.

"We've never had the political will and the resolve to treat terrorism as a real foe," observed Ted Price, a former deputy director of operations at the C.I.A. "But now we're at war."

One former C.I.A. officer argued that the agency was not organized to fight an all-out war on terrorism and that other organizations might ultimately be needed. "The C.I.A. wants to penetrate these groups, to find out about the next attack," the former officer said. "But you can never stop all the attacks because you can never hear about all of them. You can't just spy on these groups. You have to destroy them. And that's not what the C.I.A. has been set up to do."

Mr. Woolsey said in an interview on Friday that he had been steadfastly opposed to lifting the ban on assassinations until this week's attacks. "Before Tuesday, I was opposed to anything like that," said Mr. Woolsey, who also supports an end to the restrictions on the C.I.A.'s recruitment of spies with troubling background. "But like a lot of people, I've been somewhat shaken in that conviction by what happened."

The question of whether to change operating orders of the C.I.A., which operates outside the United States, is up to President Bush. The restriction on assassinations is part of a presidential executive order that could be revoked or rewritten. The rules about recruiting spies are part of the agency's internal procedures and could be revised by the director of central intelligence.

But until today, Congress, which has broad oversight powers over the intelligence community, would almost certainly have weighed in on an effort by a president to end the ban on assassinations.

For now, the C.I.A. is not pressing Congress or the White House to support any change in its rules. Beyond quick fixes, the Congressional intelligence committees are also considering more fundamental reforms, and have promised to study closely why the C.I.A. and other agencies were caught by surprise by the attacks ?the largest terrorist operation ever launched against the United States.

"I'm certain that we are going to find some significant intelligence shortfalls that contributed to this tragedy on Tuesday," Senator Graham said.

Intelligence officials defended the performance of the C.I.A. They emphasized that while the agency had failed to provide a precise warning of the attack, it had issued repeated warnings ?one as recently as August ?that the terrorism suspect Osama bin Laden and his network were seeking to attack the domestic United States.

"We have stated on a regular basis that bin Laden had declared that all U.S. citizens were legitimate targets," noted one American intelligence official. "Could we, should we, have given a tactical warning? Obviously we would have loved to."

Others note that the problems exposed on Tuesday range far beyond the C.I.A. and include the lack of coordination of the government's counterterrorism efforts, which are spread throughout several agencies.

Critics say the government fails to quickly process and analyze information that might help unravel terrorist plots. Often, crucial intelligence is found to have been sitting in the files, but is recognized as significant only in hindsight, after a terrorist incident.

For instance, United States officials noted that the C.I.A. had gathered evidence in August that Khalid al-Midhar, identified on Friday by the Federal Bureau of Investigation as one of the hijackers aboard the plane that smashed into the Pentagon, had met with suspect associates of Mr. bin Laden in Malaysia in January of last year. Subsequently the C.I.A. determined that some people at that meeting may have been involved in the plot to attack the U.S.S. Cole in October 2000.

The C.I.A. also determined that Nawaq Alhamzi, another hijacking suspect aboard the same plane, had previously traveled to the United States with Mr. al-Midhar.

In late August, the C.I.A. notified the Immigration and Naturalization Service that both men should be placed on the watch list intended to prevent their entry into the United States. The I.N.S. responded that both men had already gotten into the country, using their real names.

The F.B.I. was notified, and the bureau began to look for them. But too late.