This is precisely the kind of thing no one believes when you tell them.

Via Xan via ZDNet:

The FBI appears to have begun using a novel form of electronic surveillance in criminal investigations: remotely activating a mobile phone’s microphone and using it to eavesdrop on nearby conversations.

The technique is called a “roving bug,” and was approved by top U.S. Department of Justice officials for use against members of a New York organized crime family who were wary of conventional surveillance techniques such as tailing a suspect or wiretapping him.

... The surveillance technique came to light in an opinion published this week by U.S. District Judge Lewis Kaplan. He ruled that the "roving bug" was legal because federal wiretapping law is broad enough to permit eavesdropping even of conversations that take place near a suspect's cell phone.

Kaplan's opinion said that the eavesdropping technique "functioned whether the phone was powered on or off." Some handsets can't be fully powered down without removing the battery; for instance, some Nokia models will wake up when turned off if an alarm is set...

The U.S. Commerce Department's security office warns that "a cellular telephone can be turned into a microphone and transmitter for the purpose of listening to conversations in the vicinity of the phone." An article in the Financial Times last year said mobile providers can “remotely install a piece of software on to any handset, without the owner’s knowledge, which will activate the microphone even when its owner is not making a call.”

Nextel and Samsung handsets and the Motorola Razr are especially vulnerable to software downloads that activate their microphones, said James Atkinson, a counter-surveillance consultant who has worked closely with government agencies. “They can be remotely accessed and made to transmit room audio all the time,” he said. “You can do that without having physical access to the phone.”

Because modern handsets are miniature computers, downloaded software could modify the usual interface that always displays when a call is in progress. The spyware could then place a call to the FBI and activate the microphone—all without the owner knowing it happened. (The FBI declined to comment on Friday.)

...A BBC article from 2004 reported that intelligence agencies routinely employ the remote-activiation method. "A mobile sitting on the desk of a politician or businessman can act as a powerful, undetectable bug," the article said, "enabling them to be activated at a later date to pick up sounds even when the receiver is down."

... Surreptitious activation of built-in microphones by the FBI has been done before. A 2003 lawsuit revealed that the FBI was able to surreptitiously turn on the built-in microphones in automotive systems like General Motors' OnStar to snoop on passengers' conversations.

When FBI agents remotely activated the system and were listening in, passengers in the vehicle could not tell that their conversations were being monitored.

Malicious hackers have followed suit. A report last year said Spanish authorities had detained a man who write a Trojan horse that secretly activated a computer's video camera and forwarded him the recordings.