By Jeff Mason, Steve Holland and Trevor Hunnicutt

WASHINGTON (Reuters) -When President Donald Trump went to the Pentagon on Thursday to commemorate the Sept. 11 attacks, the ceremony took place in a courtyard rather than the memorial near the side of the building where a plane crashed and killed 184 people 24 years ago.

Two U.S. officials said the switch to a less exposed area was done out of an abundance of caution after the killing of Charlie Kirk, a prominent Trump ally and conservative activist, at a college campus in Utah on Wednesday.

Tightened security and a somber mood were evident as the White House reeled from the violent death of the 31-year-old who Trump credits with helping rally young voters to elect him.

Trump, who survived two assassination attempts himself last year, s

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