Barry Mawn was in his office downtown when he heard an airplane overhead shortly before 9 a.m. on Sept. 11. He thought the plane had broken the sound barrier and gone down in the Hudson River, but his secretary looked out her window just as a jetliner crashed into the World Trade Center.

"At that point, I thought it was an accident," Mawn recalled.

Instead, it became the focus of his work. As assistant FBI director in charge of the New York field office, Mawn has been the point man in an investigation that has involved 1,100 FBI agents and investigators from no fewer than 28 agencies.

"There really isn't a distinction between the agencies," Mawn said. "All of law enforcement came together as one. Everybody wants to be a part of this."

The collapse of the World Trade Center towers knocked out the telephone service in the FBI's New York field office, just blocks away at 26 Federal Plaza, but, within 24 hours, the FBI set up shop in the garage it typically used to house and repair its fleet. The garage would become the command center for the first four weeks of the investigation into the worst act of terrorism on American soil.

Inside the block-long, brown-brick building, agents and detectives huddled shoulder to shoulder at makeshift desks, pounding away on laptop computers and answering phones hooked up to a satellite truck outside. Homemade signs were taped to wires suspended from the ceiling: "FBI," "NTSB," and "NYPD." Federal prosecutors from the Southern District of New York also worked there. So did detectives from the New Jersey State Police.

Investigators yelled over fans circulating air warmed by the heat of around 100 computers. There were no receptionists to take phone messages, no kitchen facilities and no privacy. Hot meals were replaced by Ritz crackers, Wheat Thins and canned soup.

At the time, Mawn called it "a huge din of noise," but "functional." The FBI moved back into its Federal Plaza quarters earlier this month.

He used to be a Massachusetts schoolteacher, but in his nearly three decades with the FBI, Mawn has become an expert in terrorism. Mawn, who has snow-white hair and wears black-rimmed glasses, has been involved in several high-profile cases, including the crash of EgyptAir Flight 990 in 1999 and the bombing of the World Trade Center in 1993.

"He's the perfect guy for the job," said Tom Sheer, a former assistant director of the New York field office who has known Mawn since the 1970s, when Mawn worked on a bank robbery task force that Sheer supervised. "He's always there on the spot, on the scene. He's very hands-on."

As Mawn walked into the cramped break room that was then serving as his conference room, he seemed remarkably relaxed for someone who had worked virtually round the clock for weeks.

"Physically, it's tiring," Mawn said. "The family is concerned for me." His six grown children called daily. He rarely saw his wife.

Mawn is responsible for assimilating information from a huge and chaotic crime scene and ensuring that thousands of leads are followed. He also is in charge of gathering enough evidence to bring to justice those responsible.

"There's a certain amount of stress Barry is going through," said Lewis D. Schiliro, whom Mawn replaced last year after Schiliro retired as an assistant FBI director. "There's a focus that has to be maintained. But he is going to persevere."

After the first jetliner crashed into the trade center's north tower, Mawn headed straight to the scene and hooked up with New York Police Commissioner Bernard Kerik and other law enforcement officials.

Within minutes, another jetliner hit the south tower. Mawn stood in disbelief as chunks of concrete fell around him. "It was raining shrapnel," he said. He found a crevice to hide himself from the debris.

Soon the south tower fell. Then the north tower fell. And then Mawn's job began.

He relocated his agents from the disabled FBI building to the garage, a few miles from what had become ground zero. Within two days, phone lines were installed and computers were running. Mawn put all other investigations on hold and diverted resources to the Trade Center probe.

Soon his agents were working 16-hour days, combing through the tons of debris from the fallen towers, searching for the black boxes from the two jetliners and any documents that might contain clues about the hijackers. They were also trying to find anyone associated with the attack.

"We know what happened," said Mawn, who oversaw FBI offices in San Francisco, Newark and Boston before taking over the New York office. "We're just looking for information pertaining to individuals.

"We're taking the hijackers' names and working back, and any and all information pertaining to them and their associates," Mawn said. "Some of these people, we believe, are very key."

The investigation is personal for many in law enforcement. Dozens of New York City and Port Authority police officers were killed in the attack. So were FBI agent Leonard Hatton and John P. O'Neill, an FBI terrorism expert who had retired in August to become chief of security at the trade center.

Had O'Neill remained with the FBI, Mawn said, "he would be my principal deputy." Instead, Mawn delivered a eulogy for him.

Mawn was reluctant to discuss specific details of the investigation, but he predicted that the probe and subsequent prosecutions would take years.

Thomas V. Cash, a retired special agent for the Drug Enforcement Administration who has known Mawn since the mid-1990s, said the investigation will be taxing for Mawn. "The difficulty is there are multiple police agencies that may or may not benefit or impede what the bureau is doing," he said. "Handling the multitude of liaisons and ensuring that information is shared and exchanged is a monumental task."

Mawn also is under constant scrutiny, Cash said -- from the Justice Department, from Congress, from the White House and from the public, especially because the FBI has been criticized for mistakes in major cases, such as the investigation of nuclear scientist Wen Ho Lee and the handling of documents related to the trial of Oklahoma City bomber Timothy McVeigh.

"There's tremendous pressure," Cash said. "A lot can go wrong. But I don't know if we could have anybody better in charge than Barry."

Barry Mawn, left, head of the FBI's New York field office, spoke to reporters after anthrax cases were diagnosed in the city last week. With him are New York Police Commissioner Bernard Kerik, center, and Mayor Rudolph W. Giuliani.