NEW YORK -- A man forces his way into an apartment to rape the woman who lives there. A guy on a bicycle handcuffs and robs pedestrians, and another gropes random women on the side of the road.

In each case, men pretending to be police preyed on victims who let down their guard because they thought they were being stopped by real officers.

Officer impersonation is a nationwide crime, and it has flourished in recent years with brazen criminals sometimes going to great lengths to pull off their ruse.

"You wave a badge at someone and tell them to pull over and you'd be amazed at how many people are going to obey," said Dr. Naftali Berrill, a psychologist who runs the New York Center for Neuropsychology and Forensic Behavioral Science. "They disarm their victims by appearing to be cops."

More than a costume, the crime is about attitude.

"It takes nerves of steel to pull this off, the average person couldn't do it," Berrill said.

There are no readily available national statistics on the number of police impersonation cases, but it is enough of an issue to prompt some cities to take action.

New York City police arrest about 100 suspects annually on charges of impersonating an officer. The NYPD has a specialized unit, believed to be the only one in the country, dedicated to solving cases where a suspect impersonates an officer.

The unit analyzes statistics, uses DNA and other crime-solving techniques and even sets up sting operations, going undercover to investigate impersonators.

The majority of the cases are home invasions or robberies, like the suspect who rode around on a bike with a set of handcuffs and robbed people on the street, said Lt. John P. McGovern, who runs the command.

An impersonator is usually dressed in plain clothes and carries a small badge or identification. The badges range from generic, small metallic shields similar to what a security officer carries, to something more sophisticated and authentic-looking, police say.

That's what happened recently in Queens, where a woman was raped in her apartment by a man who banged on her door and flashed a badge, saying he was the police. A suspect was apprehended by neighbors and arrested on rape and assault charges.

New York police commissioner Raymond Kelly has said that every incident undermines real officers' ability to work.

Investigators encounter a complex criminal profile in dealing with such crimes. Some impostors are master manipulators, while others are delusional, with hero complexes.

Others impersonate police to commit crimes. Last month, men posing as officers committed a string of robberies. And drug dealers often pretend to be police to steal from other dealers, officials said.

"In some circumstances, it's a good in, it's a fast in, a safe in, and then they steal your property or commit another offense," McGovern said.

One of the biggest cases involved Lizzette Garvin, wanted in more than five states for impersonating an officer, McGovern said. For more than 20 years, she broke into gym lockers, then called the victim and pretended to be police investigating the robbery, he said. Victims turned over all sorts of personal information.

Detectives tracked her for nearly two years, until they took the case to "America's Most Wanted" in April 2007. She was arrested in Indianapolis hours after the show aired.