FBI agents are interviewing flight crews, watching security tapes and reviewing manifests as they piece together evidence the Sept. 11 hijacking leaders cased airports and possibly took a dozen test runs aboard jetliners, law enforcement officials said.

The agents have some testimony that Mohammed Atta and his accomplices may have taken pictures of airline cockpits, and surveyed the security at airport boarding gates, the officials said, speaking only on condition of anonymity.

FBI experts are still analyzing the wealth of information — from hazy post-Sept. 11 recollections of witnesses to specific airline ticket purchases.

But the evidence so far suggests "these hijackers were quiet, studious, calculating and thorough" in their operation and did so without raising suspicion, one senior law enforcement official said.

FBI Director Robert Mueller echoed those comments in a speech earlier this month.

"The September 11 terrorists spent a great deal of time and effort figuring out how America works. They knew the ins and outs of our systems," he said.

The effort to reconstruct the hijackers' preparations — which went well beyond attending flight schools — is likely to manifest itself in the trial of Zacarias Moussaoui this fall as prosecutors present evidence of the calculating nature of the hijackers he is accused of conspiring with, officials said.

The hijackers, particularly the half dozen believed to be the leaders, took numerous flights between late 1999 and their deaths in 2001. Atta traveled Europe in spring 2001. Several of the hijackers met in Las Vegas. Other traveled between flight schools.

But FBI agents have zeroed in on about a dozen flights last year in which they suspect the hijacking ringleaders took test runs, the officials said.

In nearly all the suspected trial flights, the future hijackers used their real names to book flights.

Some of the suspected test flights followed the same coast-to-coast routes as the four planes hijacked on Sept. 11, but not the same flight numbers or airlines, officials said. Most were aboard American Airlines or United Airlines jets, the officials said.

The FBI believes "they clearly were interested in transcontinental flights with lots of fuel, which would make the planes weapons of mass destruction," according to one airline industry official familiar with the passenger manifests turned over to investigators.

The agents have some testimony from flight attendants or passengers who recall men looking like the hijackers who took pictures of the cockpit aboard flights or appeared to take notes as early as last January, according to law enforcement and airline industry officials.

One pilot interviewed by the FBI, who spoke only on condition of anonymity, said agents told him the hijackers "did dry runs. At least, the pilots went on board airplanes and took notes and watched movements of crews to see what the procedures were," the pilot said.

At least one witness at Boston Logan airport has reported to the FBI seeing a man resembling Atta taking notes at the terminal gate where American Airlines Flight 11 took off a couple of days before Sept. 11 taking notes

"This man had no luggage, no briefcase — all he had was a folder," Jan Shineman, of Sudbury, Mass., said in an interview last fall. "By the time I got to the gate, I thought he was casing the flight. I thought he was observing it for a reason."