CAIRO — Benghazi-based extremist Islamic militant group Ansar al-Shariah said Saturday its leader Mohammed al-Zahawi has been killed.

Unconfirmed reports that al-Zahawi was injured or killed in an attack late last year had circulated on jihadist websites for months although the group dismissed them at the time.

The statement, posted on the group's official Twitter account Saturday, gave no details about how or when al-Zahawi was killed.

It gave condolences and vowed to take revenge and "shake the seat of power." The statement included a photo allegedly showing al-Zahawi after his death.

The group has been blamed for the September 2012 attack on the U.S. Consulate in Benghazi that killed U.S. Ambassador Chris Stevens and three other Americans.

In the attack, gunmen fired rocket-propelled grenades and stormed the mission, many waving the black banners of Ansar al-Shariah. The compound's main building was set ablaze. Stevens suffocated to death inside and another American was shot dead. Later in the evening, gunmen attacked and shelled a safe house, killing two more Americans.

The United States designated it a terrorist organization in January 2014, saying that the group emerged after the fall of longtime dictator Moammar Gadhafi. It said that along with the attack on the U.S. Consulate, the group has been involved in "terrorist attacks against civilian targets, frequent assassinations, and attempted assassinations of security officials and political actors in eastern Libya."

The United Nations also listed Ansar al-Shariah as a terrorist organization in November 2014, saying it runs training camps for foreign fighters travelling to Syria, Iraq and Mali.

Ansar al-Shariah is part of the Shura Council of Benghazi Revolutionaries, an umbrella group for the city's hard-line militias. Last spring Khalifa Hiftar, a former army general, led a unilateral offensive against extremist militias in Benghazi. On October 15, Hiftar and the elected government joined ranks against the militias.

Widespread militia violence has plunged Libya into chaos less than four years after a NATO-backed uprising toppled and killed Gadhafi.