A highly decorated, recently retired Marine captain is the third US veteran missing in action during the Ukraine war, his family and friends have said.

Grady Kurpasi — who joined the Marines after witnessing the Sept. 11 terror attacks while living in New York City — was last in contact with his loved ones on April 24, a family friend, George Heath, first told CNN.

Two days later, the Purple Heart vet left his post to “investigate” sudden “small arms fire,” and Kurpasi radioed his unit that the Ukrainian military was also opening fire, the family was told.

“That was the last time anyone heard from him,” Heath told CNN.

State Department spokesman Ned Price had confirmed Thursday that it had been alerted to a third US citizen missing in action in Ukraine.

“There are reports of one additional American whose whereabouts are unknown … Our understanding was that this individual had traveled to Ukraine to take up arms,” Price told reporters.

Don Turner, a former Marine infantryman who served with Kurpasi, told The Post that his friend had gone to Ukraine to do humanitarian work.

“This wasn’t him with a desire to go combat Russia and Putin,” Turner said, “His intention was never to go actively engage.”

But after some time in Ukraine, Kurpasi “found himself in the fight,” said Turner, who has become a spokesperson for the Kurpasi family. 

Turner said his friend had been to Bucha, the northern suburb of the Ukrainian capital where dead Ukrainians were buried in mass graves, often with their hands bound and gunshot wounds to their heads. 

He said Kurpasi also spent time in Irpin, a Kyiv suburb nearly leveled by Russian artillery.

“He definitely saw the atrocities that were being committed by the Russian military,” Turner said. 

Earlier this week, family members of Alabama veterans Alexander Drueke, 39, and Andy Huynh, 27, reporting them missing 

The State Department told Kurpasi’s wife, Kim Kurpasi, that he was being treated as missing in action because his body has not been found or identified, Heath told CNN.

Asked what the State Department had been doing to assist, Turner said, “Really not very much at all.”

But he said he understood the government’s hesitancy. 

“You’ve got to understand the situation — It’s not a state-sponsored anything,” he said of Kurpasi’s trip to Ukraine.

Turner said the family had turned up a clue, however — a cellphone trace found a device matching his phone’s unique IMEI number at the Fabrika Mall in the occupied city of Kherson a week-and-a-half ago.

Ukrainian authorities have said that dissidents, journalists and prisoners of war are being held by Russian forces in various locations around Kherson.

As Ukrainian counterattacks draw closer to the city, however, Turner said he thinks it likely the Russians will move any prisoners.

“I have a lot of hope that he’s OK,” Turner said.

“But [with] my background, I know what the possible and probable outcome could be,” he added. 

Kurpasi, a captain who had only retired from the Marines in November, had arrived in Ukraine on March 7, less than two weeks after Russia invaded.

He went to help civilians, “not to be in firefights,” said Heath, also a Marine veteran who had served under Kurpasi, who was living in Wilmington, North Carolina, before leaving for Ukraine.

“For him personally, he has a skill set that he feels he can give back,” he insisted. “He wanted to go and help the Ukrainian people. He wasn’t really planning on fighting,” Heath said.

“It just ended up being that way in the end.”

The missing veteran had been deployed four times, including three to Iraq, during his 20 years in the Corps.

He had won the Good Conduct Medal three times, the Navy and Marine Corps Achievement Medal three times, the Purple Heart medal, the National Defense Service Medal, and the Global War on Terrorism Expeditionary Medal, CNN noted.

He was a “great leader,” Heath said of the man who had been his platoon commander from 2012 to 2014.

“He always led from the front. He always took care of his Marines,” he said.

The State Department insisted Thursday that reports of the men being missing had yet to be confirmed, and they had not seen anything to suggest they were being held by invading forces.

“As of today, we have not raised this yet with the Russian Federation,” Price said.

However, he stressed that “the Russians have certain obligations” if the men had been captured.

“Members of the Ukrainian armed forces — including volunteers who may be third-country nationals incorporated into the armed forces — should be treated as prisoners of war under the Geneva Conventions,” Price stressed.

Prisoners of war must be “afforded the treatment and protections commensurate with that status, including humane treatment and fundamental process and fair-trial guarantees,” he said.

“The Russians aren’t saying that they have any Americans,” Turner said. “But we know that they do.”

With Post wires