DOVER AIR FORCE BASE, Del. (AP) — President Donald Trump is paying his respects on Wednesday at a Delaware military base as the remains of six U.S. service members killed in the crash of a refueling aircraft are returned to their families.

It will be the second time since launching the war with Iran on Feb. 28 that the Republican president will attend the solemn military ritual known as a dignified transfer, which he once described as the “toughest thing” he has had to do as commander in chief.

READ MORE: U.S. military says all 6 airmen in refueling aircraft that crashed in Iraq are dead

Upon landing in Dover, Trump was greeted on the tarmac by Col. Jamil Musa, Commander, 436th Airlift Wing, Dover Air Force Base, and Col Martha “Jeannie” Sasnett Commander, Air Force Mortuary Affairs Operations, Dover Air Force Base and headed to the ceremony — which was closed to the press.

All six crew members of a KC-135 Air Force refueling aircraft were killed last week in a plane crash over friendly territory in western Iraq while supporting operations against Iran. They were from Alabama, Indiana, Kentucky, Ohio and Washington state.

“Every person on that aircraft carried a weight most Americans will never see, and they carried it with professionalism, courage, and a level of quiet excellence that deserves to be recognized,” retired Lt. Col Ernesto Nisperos, a friend of one of those killed, said in a text message Wednesday.

The crash brought the U.S. death toll in Operation Epic Fury to at least 13 service members. About 200 U.S. service members have been injured, including 10 severely, the Pentagon has said.

READ MORE: Dignified transfer for Kentucky soldier who was 7th U.S. service member to die in Iran war

Trump last traveled to Dover Air Force Base on March 7 for the dignified transfer of six U.S. service members who were killed by a drone strike at a command center in Kuwait. He saluted as flag-draped transfer cases containing the remains of the fallen service members were carried from military aircraft to vehicles waiting to take them to the base’s mortuary facility to prepare them for their final resting place.

“It’s the bad part of war,” he told reporters afterward. Asked then if he worried about having to make multiple trips to the base for additional dignified transfers as the war continued, he said, “I’m sure. I hate to do it, but it’s a part of war, isn’t it?”

U.S. Central Command, which oversees military operations in the Middle East, said that the crash followed an unspecified incident involving two aircraft in “friendly airspace” over Iraq but that the loss of the aircraft during a combat mission was “not due to hostile or friendly fire.” The circumstances were under investigation. The other plane landed safely.

The crash killed three people assigned to the 6th Air Refueling Wing at MacDill Air Force Base in Florida: Maj. John A. “Alex” Klinner, 33, who served in Birmingham, Alabama; Capt. Ariana Linse Savino, 31, of Covington, Washington; and Tech. Sgt. Ashley Pruitt, 34, of Bardstown, Kentucky.

Klinner, who left behind a wife, a 2-year-old son and 7-month-old twins, was known for his steady command and goofy nature, as well as a willingness to help others. Pruitt’s husband described her as a “radiant” woman who lit up the room. Savino was a friend, mentee and “source of positive energy” who was proud of her Puerto Rican heritage and inspired young Latinas, said Nisperos, who is serving as spokesman for her family.

“She had had this warmth that made you feel seen, a strength that showed up in everything she touched, and a spark — that spice — that made her unforgettable,” Nisperos said. “If you knew her, even for a moment, you knew you were in the presence of someone who was going to change the world.”

The three others were assigned to the 121st Air Refueling Wing at Rickenbacker Air National Guard Base in Columbus, Ohio: Capt. Seth Koval, 38, a resident of Stoutsville, Ohio, who was from Mooresville, Indiana; Capt. Curtis Angst, 30, who lived in Columbus; and Master Sgt. Tyler Simmons, 28, of Columbus.

Koval grew up dreaming of becoming a pilot, according to his wife, who described him as a loving, generous “fixer of all things.” Angst’s family said his life was defined by service, generosity and “a genuine love for people.” Simmons loved confiding in his 85-year-old grandmother and working out with her, Sen. Jon Husted said Tuesday, when he and Sen. Bernie Moreno honored the Ohio airmen on the Senate floor.

“To the mom and dad of these three young soldiers, I can’t even process what you’re going through. I can’t even imagine the emotions that you’re feeling,” Moreno said. “Just know that America is grateful beyond words for the sacrifice that your heroic young sons made.”

Associated Press writers Holly Ramer in Concord, N.H., and Hallie Golden in Seattle contributed to this report.