Last summer, a Tampa man impersonating an undercover officer used a badge and a siren to pull over a 28-year-old woman and rape her. In January, the man, Luis Harris, 31, was convicted of sexual battery, grand theft, kidnapping and impersonating a police officer, among other charges. A judge sentenced Mr. Harris to life in prison.

Other police impersonators, police chiefs and detectives say, masquerade as officers for more benign reasons, like trying to scare or impress someone. “Usually,” Detective Baez said, “the wannabe cop outfits their vehicles with police lights and fake insignias to fulfill some psychological need.”

This happened in Chicago when a 14-year-old boy named Vincent Richardson donned police garb and walked into the Third District precinct during morning roll call in January 2009. Officers handed him a radio and told him to ride along with a female officer. The teenager even helped make an arrest.

“After four or five hours, she asks, ‘Who is this guy?’ ” recalled Jody P. Weis, who was the Chicago police superintendent at the time. “He’s in a uniform, he has a goofy badge, he doesn’t have a weapon and he’s a high school kid. It was so embarrassing.” (The embarrassment did not end there for Mr. Weis, who said he had recommended against punishing the teenager in juvenile court because no harm had been done. Three months later, the boy was arrested and charged with stealing a car. Last week, he was arrested on several weapons charges.)

Impersonating an officer is a misdemeanor in some states, though it is a felony in Florida. The charge’s severity, and punishment, increases if a criminal charged with posing as a police officer commits a felony. Several chiefs and detectives say the crime is not taken seriously enough by the justice system and the public. Often, the crime goes unreported, the police say.

“Unfortunately, there is not a lot of downside for a criminal to impersonate a police officer,” said Commissioner Edward Davis of the Boston Police Department. “You can charge them with impersonating a police officer, but that’s not a very serious crime. The way the law views this crime, it’s as an innocent or silly prank. But it has become a much more serious crime than it is perceived by the public.”

Detective Hernandez, of Biscayne Park, Fla., said: “People minimize it. They just let it go. They won’t think about how dangerous this potentially can be. They just don’t see it.”