Picured: Former SEAL Glen Doherty and Ambassador Christopher Stevens

(CBS/AP) BOSTON - A former Navy SEAL from a Boston suburb was one of four Americans who died in the attack on the U.S. consulate in Libya Wednesday.

Forty-two-year-old Glen Doherty was killed with U.S. Ambassador Christopher Stevens in Benghazi, a family friend told CBS Boston on Thursday.

Read a profile of Ambassador Christopher Stevens

"He was on security detail and he was protecting the ambassador and also helping the wounded"' when he was killed, his sister, Katie Quigly told the Boston Globe.

Doherty was a 1988 graduate of Winchester High School.

A third victim has been identified as Foreign Service Officer Sean Smith, an Air Force veteran who had worked as an information management officer for 10 years in posts such as Brussels, Baghdad and Pretoria.

The identity of the fourth American killed is still not known.