For a long time, police have urged people to lock car doors to prevent thefts of items inside. But in one of the District’s most affluent areas, thieves have been entering even locked cars, baf­fling owners and investi­gators.

Recently, police in the Northwest Washington neighbor­hoods west of Rock Creek Park said they have figured out what probably has been happening.

Thieves have been hacking into so-called keyless entry systems that permit many cars to be opened and closed remotely with the press of a button, police said this month on an email bulletin board.

Reports in Forbes and Wired magazines outline the technique.

They say an electronic device held close to a car prompts a recognition signal. Additional equipment amplifies that signal and sends it to the wireless key in the owner’s home, perhaps dozens of yards from the car.

The key then signals back, allowing the car to be entered. One author’s suggestion: Wrap the key in tinfoil when at home, blocking the radio signal.