They were four people charged with the safety of others, gunned down in a place where the nation's defense is the paramount mission for those who pass through its doors. They are being deemed heroes by some, and the cruel irony of protectors becoming targets was not lost, with President Barack Obama calling it "a heartbreaking circumstance" to lose four people who served "with great valor."

Here is a look at the Marines killed in the attack on two military facilities in Chattanooga, Tennessee:

THOMAS SULLIVAN

Ripples of grief were apparent as a stream of visitors brought flowers, food and gifts Friday to the Hampden, Massachusetts, home of Jerry and Betty Sullivan, the parents of Sgt. Thomas Sullivan. A police officer was stationed outside to keep reporters and onlookers away. Masslive.com said Sullivan, 40, grew up in Springfield, Massachusetts, served two tours of duty in Iraq and earned a Purple Heart.

His hometown mayor, Dominic Sarno, called Sullivan a man who "dedicated his life in brave service." Gov. Charlie Baker ordered flags to half-staff as he proclaimed "Terror comes home to Massachusetts." Sullivan's unit — India Battery, 3rd Battalion, 12th Marines — called him "one of our own" on its Facebook page. A giant U.S. flag and another representing the Marine Corps hung outside a Springfield restaurant owned by Sullivan's brother Joseph.

"He was our hero," read a post on the Facebook page of Nathan Bill's Bar and Restaurant, "and he will never be forgotten."

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SKIP WELLS

The mother of Skip Wells was watching news coverage of the Chattanooga shooting Thursday when a Marine Corps notification team arrived with the dreadful message.

"Every service parent, especially moms, dreads opening the front door and seeing people in uniform," said Andy Kingery, a friend who is acting as a family spokesman.

Wells was from the Atlanta area and in his early 20s. Kingery said Wells had attended Georgia Southern University but joined the Marines. He was unsure of his friend's rank or the specifics of his job, but Kingery said Wells was proud of being a Marine.

"Skip Wells died doing what he wanted to do and had chosen to do," Kingery said.

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Contributing to this report were Associated Press writers Mark Pratt in Boston; Rodrique Ngowi in Hampden, Massachusetts; Ray Henry in Atlanta; and Russ Bynum in Savannah, Georgia, contributed to this report.