The FBI's advanced surveillance methods can even activate a computer's webcam to spy on computer users — without switching on the device's green light.

The covert snooping in the case of a mystery man, "Mo," also shows how investigators can download files, photographs and stored e-mails from a computer without its owner knowing, reports The Washington Post.

According to The Post, "Mo" had threatened to blow up several people-filled facilities, including airports and colleges, if authorities wouldn't free James Holmes, who pleaded not guilty by reason of insanity earlier this year for killing 12 people in a Denver-area movie theater in 2012.

Mo, sending photographs of himself dressed in an Iranian military uniform, first contacted the FBI in July 2012, two days after Holmes was arrested. Authorities said Mo hid his location through programs that allowed him to use e-mail, video chat and an ­Internet-based phone service, but it was believed the messages were coming out of Iran, said The Post.

Federal officials haven't commented on the case, but court documents reveal the FBI's experts installed a piece of malicious software to launch into Mo's computer files when he signed on to his Yahoo e-mail account. The software would then work to gather information about his location and websites he'd visited, in hopes of tying him to the threats.

Despite the advanced surveillance techniques, Mo has not been captured and no bombs were found anywhere. But search techniques like those launched to search for him are under fire by critics who say that they gather a broad range of information that has nothing to do with the case at hand.

'You can’t just go on a fishing expedition," Georgetown University law professor Laura Donohue told The Post. "There needs to be a nexus between the crime being alleged and the material to be seized. What they are doing here, though, is collecting everything."

The FBI has been able to activate webcams for years, and has used the technique mainly to capture terrorists or for the most serious crime investigations, said Marcus Thomas, former assistant director of the FBI’s Operational Technology Division in Quantico. He is now on the advisory board of Subsentio, which helps telecommunications carriers comply with federal wiretap statutes, said The Post.