Media outlets have identified the four Marine victims killed when when a gunman attacked two military facilities in Tennessee on Thursday.

Gunnery Sgt. Thomas Sullivan, a 40-year-old reservist with 4th Marine Division, was killed in the attack on the Navy and Marine Corps Reserve Center in Chattanooga, the Boston Globe reported. Multiple media outlets identified Staff Sgt. David Wyatt, 37, Lance Cpl. Skip Wells, and Sgt. Carson Holmquist as the other victims of the deadly single-shooter rampage.

Sullivan, who grew up in Springfield, Massachusetts, recently visited his family in nearby Hampden, MassLive reported. His family is shocked by the attack and are still trying to comprehend what happened, according to the website.

Sullivan's brother is one of the owners of Nathan Bill's Bar and Restaurant in their hometown, MassLive reported. Personnel at the restaurant declined requests for comment from Sullivan's brother. On the restaurant's Facebook page, they called Sullivan their hero and said he will never be forgotten.

A large American flag is shown hanging across the front of the restaurant, along with the Marine Corps colors.

A Facebook group for Marines who've served in Sullivan's unit also posted a message about the gunnery sergeant.

"For those who have not heard yet, one of the four Marines murdered today was one of our own — Thomas Sullivan. Please keep his family in your thoughts and prayers. Semper Fidelis Zimminite!"

The FBI is leading the investigation into the multi-pronged attack. During a news conference U.S. Attorney Bill Killian said they were "treating this as an act of domestic terrorism."

Georgia Southern University posted a statement acknowledging the death of Wells, a student there from 2012 through fall 2013.

"The entire Georgia Southern University community is saddened by the news that former student and Marine Skip Wells was killed yesterday in the Chattanooga tragedy along with three fellow Marines," university officials said in a release posted to the school's web page. "The Eagle Nation offers our deepest condolences to his family and the families of those killed and wounded during this incident."

The attacks were carried out by 24-year-old Muhammad Youssef Abdulazeez, from Hixson, Tennessee. The shooter, who is now dead, first attacked the recruiting office, firing dozens of rounds into the glass doors and windows of the strip mall facility. A recruiter was injured in that attack, according to Marine officials.

Youssef Abdulazeez then attacked the Reserve center at about 11 a.m., several miles away, according to Navy officials. Four Marines, including Sullivan, were killed there. There were multiple shots fired at that facility, which is used by Navy and Marine Corps personnel. A sailor was also injured there, according to Navy officials.

Commandant Gen. Joseph Dunford tweeted a message about the attack Friday morning, asking that the fallen Marines and their families be kept in people's thoughts.

Staff writer Matthew L. Schehl contributed to this report.