(warning : this is not Ty Law)

Helmets afford a degree of anonymity for the stars of the NFL that baseball and basketball’s icons don’t experience. Just ask Ben Rothlisberger. Or better yet, the New York Times’ Karen Crouse, who does all the heavy lifting for us.

This year, NFL security’s Martin Ahlerich has investigated 23 cases involving impersonation. (Identity theft is a separate category.)

In 2004, 37 such cases were brought to the attention of the league office. That was five fewer than in 2003. How many more cases go unreported? As Martin said, he would have been none the wiser about his impersonator if not for his friend’s fortuitous timing.

“Frankly, we don’t know what we don’t know,” Ahlerich said.

Ahlerich, a 25-year veteran of the F.B.I., described a recent case that landed on his desk in which a man, claiming to be various N.F.L. players, duped four people in four cities out of $500 apiece with a story about losing his billfold and needing money to fly home to tend to a family emergency.

Each time he promised to wire the money as soon as he arrived at his destination. When he didn’t, one of the victims contacted the N.F.L. The perpetrator was caught, Ahlerich said.

Law enforcement officials can be reluctant to take on impersonation cases unless a financial loss has occurred. Many times, the only thing the N.F.L. security officials can do is deliver a stern lecture to the guilty parties.

The gall of the impersonators is matched by the gullibility of their victims. “People want to believe they’re in the presence of celebrity,” Ahlerich said. “It makes them feel good to say, ‘I hung out with so-and-so.’ ”

Jets cornerback Ty Law’s experience with impersonators has ranged from the comical to the criminal. He can laugh about the time when he played for New England that he was summoned to a Jaguar dealership to take delivery of a car he hadn’t ordered and, upon introducing himself to the sales representative, was told, “You’re not Ty Law.”

Law is less sanguine recalling the time someone purporting to be him managed to withdraw money from his bank account in Pittsburgh.

“He went to two branches within 10 to 15 minutes of each other and took me for about 20 grand,” said Law, who played 10 seasons with the Patriots before signing with the Jets in July. “It was an hour and a half from where I’m from. I’d never been to the branches in my life. He had a forged driver’s license from Ohio. He got me.”