There are growing suspicions that the attack on the U.S. consulate in Benghazi, Libya, that killed four Americans may have been a terrorist operation launched by al-Qaeda or like-minded Libyan Islamist militants in revenge for the death of a high-ranking Libyan member of al-Qaeda.

CNN reported late on Wednesday that a shadowy pro-al-Qaeda militant group, the Imprisoned Abdul Rahman Brigades, is a leading suspect in the attack. The group, named after the Egyptian “Blind Sheik,” who is serving a life sentence in the United States for conspiring to perpetrate terrorist bombings in New York City in 1993, launched a previous attack on the consulate as well as other targets in Libya.

The consulate attack may have been inspired by an appeal by al-Qaeda leader Ayman al-Zawahiri to avenge the death of his second in command, Abu Yahya al-Libi, a Libyan who was killed in a drone attack in Pakistan in June.

Eastern Libya has long been a hotbed for Islamist extremist groups and furnished a disproportionately large share of the foreign militants who flocked to Iraq to fight U.S. troops and the Iraqi government over the last decade. The presence of Islamist militants within the Libyan rebel coalition was one reason The Heritage Foundation urged caution in Libya in 2011.

As Heritage warned earlier this year, Libyan Islamists have grown stronger in post–Muammar Qadhafi Libya. In addition to various Libyan Islamist groups, al-Qaeda has reportedly exploited the lawless anarchy within Libya to gain a foothold and attract new recruits.

Although the growing presence of Islamist militants in Libya led the U.S. to dispatch surveillance drones to monitor training camps in eastern Libya, it is not clear if additional security precautions were undertaken at the consulate.

James Carafano, a defense and homeland security expert at The Heritage Foundation, noted that hard questions need to be answered about the facility’s security: “What kind of intelligence and early warning system did they have and was it adequate? Was the level of security appropriate to the assessment of level of risk that they had? Lots and lots of questions. Obviously, what security they had was insufficient.”