Washington ? The war on terror is pressing the FBI and CIA, once two solitudes, to mesh in ways their operatives could not have imagined in scrappier days.

These agencies have skirmished over everything from political favor to cubbyhole office space. Their prime missions using investigation at the FBI to solve crimes, using spies at the CIA to prevent trouble have tripped over each other at times.

On a personal level, the tweedy intelligence officer and rumpled gumshoe may be overdrawn stereotypes, but ones that betray a different world view and way of doing things.

“It’s been a rocky relationship,” said Loch Johnson, an author on intelligence agencies who teaches political science at the University of Georgia. “These days it’s very uneven.”

The two agencies have been working systematically since the mid-1990s to combine their strengths and communicate more effectively than they did when the Potomac River between their headquarters stood as a mighty gulf.

Their long rivalry and recent collaborations will be one of the subjects for exploration in joint Senate and House intelligence hearings that begin this week, called to find out why the Sept. 11 attacks were not foreseen by either agency.

The effort to spur cooperation is most apparent at senior levels, where the CIA in Virginia and the FBI in Washington have placed top officers in the counterintelligence and counterterrorism operations of one another.

In the aftermath of Sept. 11, FBI Director Robert Mueller or a deputy has joined the daily CIA intelligence briefings given to President Bush, sitting in on the portion that deals with counterterrorism.

The CIA is sending 25 analysts to work with domestic FBI agents in an expansion of joint operations and officer swaps that have been taking place for several years.

“We have to do a better job of collaborating,” Mueller said during a week in which he acknowledged widespread FBI shortcomings and set out steps to fix them.

The unevenness Johnson referred to was evident before and after Sept. 11.

When Minnesota FBI agents took their concerns about alleged terrorist Zacarias Moussaoui directly to the CIA, they got their wrists slapped by superiors for going around their headquarters, FBI officer Coleen Rowley said in her whistle-blowing memo to Mueller.

The FBI did not tell the CIA about a pre-Sept. 11 memo from an agent warning of Arabs taking flying lessons.

John Deutch, CIA director in the mid-1990s, says some tensions between the agencies are natural. CIA success tends to be measured in foiled plots the public does not know about; the FBI’s, in bad guys behind bars.

The fear, he said, is that an FBI tip to the CIA about an investigation might risk spoiling a prosecution. A CIA tip to the FBI could compromise a carefully nurtured source.