NORFOLK, Va. » The sailor who was slain during a shootout aboard a guided-missile destroyer at a Virginia base saved another sailor’s life by jumping between her and a civilian gunman who was trying to board the ship, Navy officials said Wednesday.

The Navy identified the slain sailor as Petty Officer 2nd Class Mark Mayo during a news conference, saying he acted heroically when the civilian approached the USS Mahan shortly after 11 p.m. on Monday. The civilian gunman, who was also killed in the shootout, hasn’t been identified.

The truck driver used a legitimate transportation worker’s credential to get onto Naval Station Norfolk, although officials say he didn’t have a reason to be on the world’s largest naval base on Monday night. The Navy hasn’t said why the truck driver was trying to get onto the ship, but officials have ruled out any link to terrorism and say there is no evidence to indicate a pre-planned confrontation.

He parked his tractor-trailer cab near Pier 1, was able to walk onto the pier and began heading up a ramp toward the USS Mahan when he was confronted by Navy security, said Mario Palomino, the Naval Criminal Investigative Service special agent in charge of the Norfolk field office.

The man then got into an altercation with a female petty officer and disarmed her, Navy officials said. Palomino said Mayo stepped over the disarmed officer and fired his weapon at the assailant. He was the installation’s chief of the guard and came to help once he saw the civilian board the ship.

Multiple pistol rounds were fired between the gunman and Navy security forces responding to the scene, Palomino said. The Navy has said previously that the truck driver fired the shot that killed Mayo.

The base’s commanding officer, Capt. Robert Clark, said Mayo’s actions to protect the disarmed officer were extraordinary.

""He basically gave his life for hers," said Clark said during a news conference.

Mayo, a 24-year-old master-at-arms from Hagerstown, Md., joined the Navy in 2007 and had previously been stationed in Bahrain and Rota, Spain before coming to the Norfolk base in 2011.

On Wednesday, the Navy appointed Rear Adm. Jeffrey Harley to lead an investigation into the circumstances surrounding the shooting. Harley’s role will be different from that of criminal investigators, and will include a review of base procedures, according to the Navy.

To get onto the base, civilians must be escorted or have identification that allows them to be there. Clark said the gunman was found with a TWIC card, a transportation worker’s credential issued by the Transportation Security Administration to personnel such as truck drivers who require unescorted access to secure areas.

He said investigators were looking into why the truck driver was allowed onto the base when he didn’t have a reason to be there that night. The base is reviewing its security procedures.

"If I find the procedures were not adhered to, I will take immediate and appropriate corrective action. The security of this installation is my highest priority," Clark said.

In addition to the base’s entry gates, all 13 piers also have their own security.

The shooting on the Mahan comes about a month after the Navy held anti-terrorism and security exercises on bases across the U.S., including a drill on simulating a shooting at Naval Station Norfolk. And it follows a September shooting at the Washington Navy Yard, in which a gunman — identified as a contractor and former Navy reservist — killed 12 civilian workers before being shot to death.

The Norfolk base was briefly on lockdown after the shooting but resumed normal operations aside from the Mahan’s pier Tuesday. On social media, many of those who had connections to the ship or its sailors changed their profile pictures to the ship’s emblem.

Naval Station Norfolk covers more than 6,000 acres and is the home port for 64 ships. About 46,000 military members and 21,000 civilian government employees and contractors are assigned to the base and its ships, according to the Navy figures.

Friends and family remembered Mayo as an above-average student with an infectious smile who joined the Navy for its educational opportunities.

Born in Washington, D.C., Mayo moved with his family in 1998 about 70 miles northwest to the Hagerstown area because his mother felt it was a safer environment. They initially lived in public housing but moved in 2005 into their own home.

"He pulled himself up by his bootstraps. He worked really hard," said Williamsport High School guidance counselor Randy Longnecker. The family

Mayo’s parents Sharon Blair and Decondi Mayo said their son’s actions reflected his strong, caring, nature.

"He protected people. He was a protector," Blair told reporters at her home.

Mayo’s older brother Timothy Borum Jr., 29, said he wants to know how the shooting could have happened on the Navy’s largest base.

"I always felt that he was safe on a military base, always felt that he’d be protected," Borum said.

Associated Press writer David Dishneau contributed to this reported from Hagerstown, Md.