The US official, who spoke on condition of anonymity, could not confirm the name of the dead commando, but US media identified him as 42-year-old Glen Doherty.
The former member of the elite SEAL unit had been working on a mission to track down shoulder-launched surface-to-air missiles in Libya, according to ABC News.
US military and intelligence officials have warned that thousands of the weapons, so-called MANPADs, were unaccounted for after Libya’s former dictator, Moamer Kadhafi, fell from power.
The former SEAL described his job in an interview with ABC last month, saying he had traveled across the country chasing leads and then once the weapons were found, his team would destroy them on the spot, the American television network said.
US officials previously confirmed Tuesday’s assault by armed militants claimed the life of the ambassador to Libya, Chris Stevens, and a State Department information management officer, Sean Smith.
A fourth victim has not been identified and it was possible the victim was also a retired member of the military working as a security contractor, officials said