The FBI has developed advanced surveillance techniques that give it the power to covertly activate Web cams to spy on unsuspecting computer owners.

Tech savvy G-men can remotely turn on cameras that transmit real-time images to investigators — without triggering the light that shows the camera is in use, according to The Washington Post.

The FBI can also burrow into a suspect’s computer and download files, photographs and stored e-mails.

The new snooping capabilities came to light during an investigation of a mysterious man named “Mo’’ – who threatened to blow up a building filled with innocent people unless authorities free Colorado movie-theater shooting suspect James Holmes.

He also threatened to bomb a jail, a hotel, three colleges and two airports.

No bombs were found at the targets he mentioned.

He first contacted federal authorities in July 2012. It’s not clear how long Mo and the FBI were in touch.

The paper said he sometimes used an untraceable e-mail, other times an encrypted phone.

Mo even sent the FBI pictures of himself fashionably decked out in an Iranian military uniform.

The FBI, frustrated in its attempts to track him down, used special software that would install itself in Mo’s computer when he opened his e-mail.

It was designed specifically to help agents track his location and his movements.

But the software never worked as designed, the paper said, and Mo remains at large.

The feds had gotten permission to install the software from a Denver judge.

The agency tried to use it on at least one other probe, but a Houston judge described the method as “extremely intrusive’’ and probably unconstitutional — and shot it down.

The FBI has had the capability to sneak into computers’ Web cams for several years, a former employee of the agency told the Washington Post.

It was not clear how many times it tried to do it, but the paper’s source said the FBI has used its tool mainly in terrorism and the “most serious’’ investigations.

The technology is highly controversial.

“We have transitioned into a world where law enforcement is hacking into people’s computers and we have never had public debate,” Christopher Soghoian of the American Civil Liberties Union complained to the paper.