The FBI and other federal agencies are going undercover on Facebook, LinkedIn, MySpace and other social networks with phony profiles to gather information and communicate with suspects, according to an internal Justice Department document.

FBI agents, for example, have used Facebook to determine the whereabouts of a fugitive. Other investigators can check alibis by comparing stories a suspect tells police with their tweets sent at the same time .

A civil liberties group, The Electronic Frontier Foundation, obtained the 33-page document after suing the Justice Department.

It plans to make it public today on its website, http://www.eff.org.

According to the Associated Press, the document says Facebook is "often cooperative with emergency requests" from federal investigators while Twitter's lawyers demand a warrant or subpoena before it will turn over customer information.

The Justice Department says such covert investigations are legal and are governed by as-yet undisclosed internal rules, the AP says, quoting from the document.

In one section, Justice discusses its own lawyers, saying social networks are a "valuable source of information on defense witnesses."

"Knowledge is power," the document says. "Research all witnesses on social networking sites."

At the same time, the document warns law enforcement officials themselves to think prudently before adding judges or defense counsel as "friends" on these services.

See a related posting here on how the Italian police used Facebook to track down and capture one of the country's most wanted mafia suspects.

(Posted by Doug Stanglin)