MIAMI >> A black BMW flashing red and blue lights suddenly filled Alexandria Armeley’s rear-view mirror one evening last month. At a stoplight, the BMW’s driver pulled up next to her, waved a gold badge and told her “I’m a cop.”

But Armeley was suspicious. Before she pulled over, she called her stepfather, Alex Hernandez, a police detective in Biscayne Park, Fla., who warned her that the man was probably not a police officer. Speed away, he told her.

A terrified Armeley took off and was chased by the BMW for several miles through southern Miami-Dade County. Hernandez had jumped in his car to help and eventually caught up to them.

So the real officer arrested the fake officer, whose name is Daniel A. Barros. Asked why he had tried to pull over Armeley, a 23-year-old college student, Barros, 22, told officers, “She was speeding.”

The BMW 7 Series car, outfitted with police lights and a siren, was “lit up like a Christmas tree,” Hernandez recalled about the midnight encounter. “There are a lot guys walking around with phony badges, but this guy had the whole works. Who knows what he would have done if he had gotten my stepdaughter to stop?”

Barros is facing several charges in the case, including impersonating an officer.

As long as police officers have worn uniforms and carried badges, criminals have dressed like them to try to win the trust of potential victims. Now the impersonators are far more sophisticated, according to nearly a dozen city police chiefs and detectives across the country.

In South Florida, seemingly an incubator of law-breaking innovation, police impersonators have become better organized and, most troubling to law enforcement officials, more violent. The practice is so common that the Miami-Dade Police Department has a Police Impersonator Unit.

Since the unit was established in 2007, it has arrested or had encounters with more than 80 phony officers in Miami-Dade County, and the frequency has increased in recent months, said Lt. Daniel Villanueva, who heads the unit.

“It’s definitely a trend,” Villanueva said. “They use the guise of being a police officer to knock on a door, and the victim lowers their guard for just a second. At that point, it’s too late.”

He added that part of the problem was that it was easy for civilians to buy “police products,” like fake badges, handcuffs and uniforms. “The states need to lock this down and make impersonating a police officer a more serious crime because we’re seeing more people using these types of these things to commit more serious crimes,” he said.

Detective Javier J. Baez of the Miami-Dade Police Department said, “These types of crimes here in Miami typically have a nexus to drugs.”

Increasingly, fake police officers are pulling off crimes together, the authorities say.

One evening three weeks ago, three men in police uniforms knocked on the door of a home in southwest Miami-Dade County.

When the home’s owner, Jose Montoya, opened the door, the men barged in and yelled, “Police, police! Get down, get down!” The men tied up Montoya, his wife and their toddler and then spent hours ransacking the house, the authorities said. They beat up Montoya, who was treated at a nearby hospital, and stole cash, jewelry and several weapons, the police said.

Before leaving, the robbers warned Montoya and his family not to call the police, the authorities said, or they would return and kill them.

Some police impersonators commit violent crimes like home invasions, car-jackings, rapes and, rarely, murders.

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 Harris to life in prison.

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.”

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.”

Some law enforcement officials said the public did not take these types of incidents seriously because of the types of cases often highlighted by the news media. People charged with impersonating police officers are often portrayed as befuddled, hapless and harmless.

In March, a motocross champion was arrested in Orlando, Fla., and charged with impersonating a police officer. The man, James Stewart Jr., 25, tried to stop another car using red and blue lights, the Florida Highway Patrol said. The car that he tried to stop contained two off-duty troopers.

In October in Boca Raton, Fla., Andrew Novotak, in his white Crown Victoria with flashing green lights, pulled over motorists and quizzed them about whether they had been drinking alcohol, the police said.

When the police questioned him, Novotak was wearing a police badge and carrying a loaded gun. He also had a German shepherd in his back seat, which he insisted was a police-trained dog. After arresting him, officers said they smelled alcohol on his breath. He was charged with impersonating an officer and driving under the influence.

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