WASHINGTON - US State Department officials suspected that two Libyan guards hired by its own security contractor were behind an April incident in which a homemade bomb was hurled over the wall of the special mission in Benghazi, according to official emails obtained by Reuters.

But the men, who had been taken into custody the day of the attack, were released after questioning by Libyan officials because of a lack of "hard evidence" that could be used to prosecute them, the State Department emails show.

"Amazing," wrote Eric Nordstrom, then the regional security officer with the US Embassy in Libya, describing the obstacles in prosecuting the suspects.

The April 6 incident involving an improvised explosive device (IED) was a troubling precursor to the Sept. 11 attack on two U.S. government compounds in Benghazi, which killed four Americans, including U.S. Ambassador Christopher Stevens.

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