CAIRO, (Reuters) – Egyptian authorities are holding a man whose militant group is suspected of links to an attack in Libya in which the American ambassador was killed, U.S. officials in Washington and a security source in Cairo said on Saturday.

“They arrested Mohamed Gamal,” said one U.S. official.

The security source in Cairo, giving the man’s full name as Mohamed Gamal Abu Ahmed, said the suspect was being questioned about an alleged role in the attacks in Benghazi in September in which Christopher Stevens, the U.S. ambassador, died along with three other American officials.

U.S. officials said Abu Ahmed’s group was suspected of a role in the violence but they were not sure whether he personally was involved.

Abu Ahmed, released from an Egyptian prison in 2011 after the uprising that toppled Hosni Mubarak, was originally arrested three weeks ago on charges of leading a militant cell that comprised members from Egypt and Libya.

Egyptian security sources had earlier said the Abu Ahmed group, which they said had al Qaeda backing, was planning attacks in Egypt and abroad. Others linked to the group were arrested in October, including a Tunisian and a Libyan.