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The National Football League's unprecedented new effort to protect its image by cracking down on loutish behavior is making some of the league's 1,952 players a little nervous.

This sweeping new personal-conduct policy, which was announced before the 2007 season, allows the NFL to quickly and summarily fine and suspend players; not just for committing crimes, but for any act that's deemed harmful to the NFL's "integrity and reputation." To guard against these unpredictable suspensions (there have been 10 so far), NFL teams are hiring former police officers and FBI agents as security chiefs, ordering up extensive background checks, installing video-surveillance systems in locker rooms, chasing down rumors and sometimes forbidding players from talking to the press.

During a recent road trip, the San Diego Chargers not only conducted bed checks, but placed guards in the hotel hallways to make sure players didn't sneak out. The Seattle Seahawks have declared an entire downtown entertainment district off-limits, and the Denver Broncos have begun sending a former cop to local nightclubs on weekends to make sure the players behave.

The increased scrutiny has taken a toll on some players, including Broncos defensive tackle Marcus Thomas. Last year, after the policy was announced, Mr. Thomas had called his agent in a panic: He said he was convinced he was being followed by "a white man in sunglasses" who had been sent by the NFL. A league spokesman says NFL security did not follow Mr. Thomas.