WASHINGTON (AP) — President Donald Trump is set to pay his respects on Wednesday at a Delaware military base when 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 on 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.

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.

The crash raised the U.S. death toll in Operation Epic Fury to at least 13 service members, with approximately 200 U.S. service members injured, including 10 suffering severe injuries, according to Pentagon reports.

Trump last visited 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.

“It’s the bad part of war,” he told reporters afterward, expressing the emotional weight of these visits. “I hate to do it, but it’s a part of war, isn’t it?”

U.S. Central Command reported that the crash followed an incident involving two aircraft in “friendly airspace” over Iraq, clarifying that the aircraft loss during a combat mission was “not due to hostile or friendly fire.” The matter is under investigation.