A Winchester Massachusetts native, who was a former Navy SEAL, is one of four Americans killed in Libya by a mob of Islamic extremists, his sister said today.

Glen Doherty was 42 years old and was working for a private security company when he was killed with US Ambassador John Christopher Stevens in the American consulate in Benghazi Wednesday during what may have been a coordinated terrorist attack.

“He was on security detail and he was protecting the ambassador and also helping the wounded’’ when he was killed, said his sister, Katie Quigly of Marblehead. . .