TRIPOLI, Libya — Libyan authorities have arrested a man suspected of taking part in the Islamic State group’s beheading of 21 Egyptian Coptic Christians in Libya in 2015, an official said Thursday.

Chief Prosecutor Al-Sadiq al-Sour told reporters the beheadings took place behind a seaside hotel in the coastal city of Sirte.

“The individuals involved in the beheading are known to us by name,” he said, adding that search operations are underway. He said authorities have also learned where the bodies were buried.

The killings, documented in a grisly video released online by the extremists, shocked Egypt and led it to launch punitive airstrikes.

Hundreds of IS militants are believed to have fled Sirte into the Libyan interior after their defeat last year by militias loyal to a U.N.-backed government in Tripoli, the capital.

Al-Sour said security forces have also detained militants who were involved in the 2012 attack on the U.S. Embassy in Benghazi, which killed Ambassador Chris Stevens and three other Americans. He did not provide further information on the number of attackers arrested or their role in the hours-long assault by dozens of gunmen.

The 2012 attack took place before the emergence of the Islamic State group and was blamed on other extremists. Al-Sour said many of the attackers later joined IS.

Al-Sour said at least 800 arrest warrants have been issued for suspected IS militants involved in scores of attacks across the country.

Al-Sour said the extremists have mainly entered Libya from Sudan, but have also come in through other routes. Neighboring Egypt and Tunisia have stepped up surveillance along their borders.

Libya was plunged into chaos following a NATO-backed 2011 uprising that toppled and later killed longtime dictator Moammar Gadhafi. It is split between rival parliaments, governments and militias in the east and west.