Benghazi attack
A protester reacts as the U.S. Consulate in Benghazi is seen in flames during a protest by an armed group said to have been protesting a film being produced in the United States on Sept. 11, 2012. Reuters/Esam Al-Fetori

Faraj al-Shibli, a suspect in the September 2012 attack on the U.S. embassy in Benghazi, Libya, was found dead Monday while in the custody of local police in the northeastern city of Marj, CNN reported.

Al-Shibli, who is believed to have been linked to al Qaida in the Arabian Peninsula, or AQAP, was taken into custody by the Libyan police in Marj two days ago, but it is unclear what caused his death, CNN reported citing sources, adding that the FBI had questioned him last March in the presence of Libyan authorities and had sought to interview him again at the time before his subsequent release.

AQAP was the first of the parent group's many affiliates to react to the attack, which killed the U.S. ambassador and three other Americans. AQAP also reportedly released a statement at the time stating that the attack was intended to avenge the June 2012 death of Abu Yahya al-Libi, a senior al Qaida militant. Although Shibli’s presence at the American compound was not confirmed, Libyan police had taken him into custody last March suspecting his involvement in the attack, and U.S. officials considered him a serious threat.

"We think there are more than a dozen people involved in this." Rep. Jason Chaffetz of Utah said on Shibli’s arrest last March, according to CNN, adding: "The idea that potentially we have somebody ... it's obviously positive news."

Last month, U.S authorities took into custody another suspect, Ahmed Abu Khatallah, suspected to be the ringleader of the attack. Earlier this month, the U.S. House of Representatives also approved a select committee to investigate the Benghazi attacks and is estimated to spend $3.3. million on the process.

Khatallah has pleaded not guilty of conspiring with the terrorists, but "gave voluntary statements corroborating key facts,” according to the government filing on the case, posted on Politico's website.