The failure to detect the September 11th plot has heightened calls for changes in the U.S. intelligence apparatus. Both the FBI, responsible for domestic intelligence, and the CIA, the foreign intelligence arm, have come under criticism at the hearings of the commission investigating the terrorist attacks. However, changing the way U.S. intelligence agencies do business and interact with each other won't be easy.

The September 11th plot was hatched overseas and nurtured in the United States, but planning went undetected by both the CIA abroad and the FBI at home. President Bush - among others - has said that failure means intelligence reform, especially on the domestic side, might be overdue.

What form that will take is not clear, but all agree that the domestic and foreign sides of the intelligence establishment have to work more closely together. Todd Masse, a specialist in domestic intelligence and counterterrorism at the Congressional Research Service, says fighting terrorism demands closer cooperation.

"I think there is a need for closer relationship of those two interrelated disciplines because, let's face it, terrorism is both a crime and a threat to national security," he said. "So in order to address that, you have to bring together and bring to bear all of the different powers that the United States has, whether that be law enforcement, intelligence, diplomatic, and economic."

That will not be easy. Over the years, the FBI - which is essentially a law enforcement agency with a domestic intelligence function - and the CIA have distrusted each other. In effect, say former officials from both agencies, the cops don't like the spies, and the spies distrust the cops. The relationship was further poisoned by some high-profile instances of espionage at both agencies, such as that of Aldrich Ames at the CIA and Robert Hanssen at the FBI's counterintelligence division.

Unlike in many other countries, domestic and foreign intelligence functions have been kept distinctly separate in the United States. There was in fact an uproar at the creation of the CIA in 1947 from people who felt it would create an "American Gestapo." For that reason, the CIA is by law forbidden to conduct operations in the United States, and the domestic intelligence function has stayed with the FBI.

John McGaffin has a unique perspective on the issue. He has at different times worked at senior positions in both agencies, serving as assistant deputy director for operations at the CIA and as a special adviser to the FBI director. He said that both agencies have tried what might be called "cross-pollination" - sending officers to work at each other's agency, but he said the cultural gap between the spies and the policemen was too great.

"It used to be known as 'hostage exchange,' where you'd send a senior person from both sides and they would theoretically be fixing the problem," he recalled. "We've gone from that to putting people from both sides in the other organization really doing senior-level and middle-level work and we still have the problems. September 11 happened despite the fact that there are a lot of FBI people at CIA, and CIA people at FBI."

Noting what has been described as a "cultural gap" between the police and the espionage mindset, some observers have called for a new domestic intelligence agency, but there are many, especially former and current FBI officials, who believe that would be grave mistake, as former FBI director Louis Freeh testified to the 9/11 commission on Tuesday.

He said, "I don't think in the United States we will tolerate very well what is in effect a state secret police, even with all of the protections and the constitutional entitlements that we would subscribe it with."

A just-issued Congressional Research Service paper lays out five options for reforming domestic intelligence, ranging from beefing up the FBI's domestic intelligence capability to stripping the FBI of all intelligence duties.

In his news conference Tuesday, President Bush did not commit himself to any particular change, saying he was open to hearing whatever options the 9/11 commission and Congress come up with.