NYPD walks international beat in search of terrorists.

New York City police are not just patrolling the five boroughs in search of terrorist cells. They also have deployed investigators around the world, according to one of the city's top counter-terrorism officials.

The department conducts its own intelligence analyses and has detectives dispersed around the world, said Michael Sheehan, deputy commissioner of counter-terrorism at New York's police department. They keep track of potential attackers and "how they may show up in New York City," he said. The police department is going to increase its presence, Sheehan added, especially in the Arab world.

At the same time, police have stepped up domestic efforts to monitor radical groups and harden potential targets. "We have to improve the defensive posture of the city, but at the same time, not allow barriers to surround every building," Sheehan said.

Because al Qaeda member movements have been under the microscope since September 2001, the organization is seeking to recruit local operatives, preferably with U.S. citizenship, said Sheehan. "There are pockets of individuals that we need to worry about in New York City," he said at this year's Eisenhower national security conference. "They discuss jihad. They discuss violence."

Often these groups are involved in extremist movements, but are not necessarily defined as terrorist organizations, he explained. It is almost impossible to know whether there is "a Mohamed Atta-like cell in our midst," Sheehan said.

Atta, the head of the September 11 cell, was instructed by al Qaeda senior leadership not to go near mosques or extremist group meetings in order to keep a low profile, Sheehan recalled. The Madrid cell also employed this subtle operational profile, he said.

Thwarting attacks means establishing constant surveillance and protective measures without crippling daily life of the world's best-known metropolis. "We have to improve the defensive posture of the city, but at the same time, not allow barriers to surround every building," Sheehan said.

In order to deter potential attackers, at least initially, the department has sent "highly visible surges of police around the city," he said. The department also has its own tabletop training for chemical and biological defense, said Sheehan, and officers have become more proficient in that area. "We operate 24/7 under the assumption that we are being targeted," Sheehan noted.

New York increasingly has to deal with a "troubling" international trend, which appeared in the Madrid train bombings back in March, Sheehan said. Attackers can bypass the professional training offered in specialized camps that previously were good locations to identify terrorist operatives.