(CNN) -- A pro-al Qaeda group responsible for a previous armed assault on the U.S. Consulate in Benghazi is the chief suspect in Tuesday's attack that killed the U.S. ambassador to Libya, sources tracking militant Islamist groups in eastern Libya say. They also note that the attack immediately followed a call from al Qaeda leader Ayman al-Zawahiri for revenge for the death in June of Abu Yahya al-Libi, a senior Libyan member of the terror group. The group suspected to be behind the assault -- the Imprisoned Omar Abdul Rahman Brigades -- first surfaced in May when it claimed responsibility...