(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.

Glen Doherty, 42, was killed with U.S. Ambassador John Christopher Stevens in Benghazi, a family friend told CBS Boston on Thursday.

"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.

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The four Americans were killed when a group of U.S. employees went to the consulate to try to evacuate staff as the building came under attack by a mob wielding guns and rocket propelled grenades. Stevens is the first U.S. ambassador to be killed in an attack since 1979, when Ambassador Adolph Dubs was killed in Afghanistan.

"The mission that drew Chris and Sean and their colleagues to Libya is both noble and necessary, and we and the people of Libya honor their memory by carrying it forward," U.S. Secretary of State Hillary Clinton said.

U.S. officials said some 50 Marines were being sent to Libya to reinforce security at U.S. diplomatic facilities.