BENGHAZI—The head of Libya's new national congress has blamed al Qaeda-linked militants for planning Tuesday's deadly attack on the U.S. Consulate in the eastern Libyan city of Benghazi, raising further questions about the motive behind the mayhem that killed four Americans.

U.S. officials told The Wall Street Journal on Friday that they are investigating indications that a local group of Libyan militants, Ansar al Sharia, held a series of conversations Tuesday with al Qaeda extremists about the assault that day on the consulate,...