FBI Terror Expert Sees Job Evolve

"You took it personally," he says of the terrorist attack. "That's why, maybe, I still have this passion. To me, it's like I'm in the military. That's what the bureau is to me. I'm fighting a war."

Over the next several years, as he was promoted to the No. 2 job in the Washington office's counterterrorism division, Perren built a crisis response branch. He oversaw the expansion of the joint terrorism task force to include 34 local and federal agencies throughout the region. He also established an intelligence squad to support day-to-day investigations and forecast threats.

Perren declines to say how many people work in the counterterrorism division today, and he will not provide a breakdown of their cases, citing security regulations. But he confirmed that the number has increased since the Sept. 11 attacks. "There is no lead left unaddressed. It's why we have so many cases," he said. "You have a lead, you run that thing into the ground."

Some tips go nowhere, and those cases are closed within days; other cases stretch into years.

Arab and Muslim organizations have criticized some of the FBI's efforts as overly aggressive. Perren acknowledges that when agents began questioning immigrants after Sept. 11, 2001, many feared that "we were going to take them away in the middle of the night."

"We had to show them to trust us," he says. The FBI has tried to build bridges to the Arab and Muslim community, but it has had mixed success.

In 2005, Perren spent six months running the FBI operation in Iraq. It was a powerful experience, he recalls, supervising agents and analysts investigating roadside bombings and seeking intelligence about threats to U.S. forces.

"To watch them in these extreme conditions, working their hearts off, up to 20 hours a day . . . it was a real privilege," he says.

Perren's father died while Perren was in Iraq. He kept the news to himself so the troops would not be aware of his grief. "You can't lose faith in your on-scene commander," he explains. "Game face on."

His office is a study in machismo, decorated with police badges and photos from Iraq and the Pentagon. But there is a hint of a softer side, a line of statues of police protecting children. They are gifts from his only child, John. Next month, the 23-year-old will graduate from the D.C. police academy. He will wear his father's old badge number.

"I'm very proud of him," says the elder Perren, and a smile spreads across his face.

But there is little time for sentimentality.

"I keep getting buzzed," Perren says, pulling out his BlackBerry. "Let me make sure we're still okay."