Nearly a decade after Sept. 11 forced New York to rethink its defenses against a terrorist attack, officials see the city's sprawling subway system as a prime target.

Subway bombings from Minsk to Madrid have sharpened awareness that the New York City subway, which carries 5.2 million riders on the average weekday, could be vulnerable without stepped up safety measures. So officials have worked to bolster the system's defenses, employing everything from bag checks to security cameras to counterterrorism training for police officers.

It's really a potentially very vulnerable environment - one that you can't totally protect, William Bratton, a Kroll security firm executive who's headed New York and Los Angeles police departments and was chief of the New York City transit, told The Associated Press. It's a unique challenge.

Officers patrolling the subway now rely on more than 30 bomb sniffing dogs, a network of security cameras providing live feeds of highly trafficked stations and motion detectors monitoring ventilation systems. Messages instructing passengers to be alert -- if you see something, say something -- pepper the trains.

In addition to new technology, officers are being trained to make keeping an eye out for potential terrorists part of their daily duty. That includes instructing NYPD officers on telltale warning signs that should spur suspicion, like a person walking stiffly and perspiring profusely. The police conduct drills in which undercover officers attempt to sneak decoy bombs into the subway, patrol tunnels running under the East River and perform tens of thousands of random bag checks a year.

We have a lot of ground to cover, Police Commissioner Raymond Kelly said. He added that the NYPD is constantly looking for new ways to make our presence seen and felt in different ways, giving would-be terrorists and common criminals cause to think twice.

Fears of a terrorist attack are not just abstractions. In 2008, authorities warned New York law enforcement of a plausible but unsubtantiated threat of an al-Qaeda plot to strike the subway during the holiday season, and in February of 2011 an Afghan immigrant named Najibullah Zazi pled guilty to planning a martydom operation in which he bombed the subway on the anniversary of the Sept. 11, 2001 attacks.