US senators called for a comprehensive probe into security protocols at US facilities worldwide Friday after protests that left four Americans dead and several missions torched or breached.

Two different sets of lawmakers wrote letters to US officials, including Secretary of State Hillary Clinton, seeking investigations into security operations, and any necessary recommendations on how to boost security and prevent similar attacks from occurring in the future.

Senate Homeland Security Committee Chairman Joe Lieberman, an independent who caucuses with Democrats but is hawkish on national security, joined the panel’s top Republican Susan Collins in urging State Department officials to focus on the US consulate in the eastern Libyan city of Benghazi.

Ambassador Chris Stevens was killed with three other American workers at the facility on Tuesday, the anniversary of the 9/11 attacks.

The pair also sought to determine whether the Benghazi consulate, as an “interim facility,” was subjected to the same stringent security requirements as permanent State Department installations, and whether there had been sufficient vetting of local nationals hired to defend the embassy.

They requested an examination of operational security connected to Stevens’s schedule.

US officials are investigating the possibility that the assault was a plot by Al-Qaeda affiliates or sympathizers, using a protest against an American-made Internet video that denigrates Islam and its Prophet Mohammed as a cover to carry out a coordinated revenge attack 11 years after 9/11.

Other protests have followed on US installations and on the streets in other Muslim-majority countries in North Africa, the Middle East, Africa and Asia.

Democrats Barbara Mikulski and Bill Nelson and Republicans Dan Coats and Roy Blunt, all members of the Senate Select Committee on Intelligence, wrote Clinton seeking a “thorough interagency security review” of protocols, including use of locals in providing perimeter security at diplomatic posts.

The State Department said officials have reviewed security at all overseas US missions, including in Benghazi, prior to the anniversary of the September 11 attacks.

It also dismissed reports that Stevens had eschewed tighter security, saying “normal security precautions” were in place at the time of the attack.