WASHINGTON –– The man who called himself “Mo” had dark hair, a foreign accent and — if the pictures he e-mailed to federal investigators could be believed — an Iranian military uniform. When he made a series of threats to detonate bombs at universities and airports across a wide swath of the United States last year, police had to scramble every time.

Mo remained elusive for months, communicating via ­e-mail, video chat and an ­Internet-based phone service without revealing his true identity or location, court documents show. So with no house to search or telephone to tap, investigators turned to a new kind of surveillance tool delivered over the Internet.

The FBI’s elite hacker team designed a piece of malicious software that was to be delivered secretly when Mo signed on to his Yahoo e-mail account, from any computer anywhere in the world, according to the documents. The goal of the software was to gather a range of information — Web sites he had visited and indicators of the location of the computer — that would allow investigators to find Mo and tie him to the bomb threats.

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