Footage of a group of teenagers - many wearing Make America Great Again caps - taunting a Native American man in Washington DC has drawn criticism.
The teenagers, students at Kentucky's Covington Catholic High School, are seen mocking Omaha elder Nathan Phillips as he sings and drums.
The students were taking part in an anti-abortion rally on Friday, while Mr Phillips came for an Indigenous Peoples' March.
The school apologised to Mr Phillips.
The footage of the incident went viral on social media.
A number of users said the youths' behaviour was "appalling" and their parents and school "should be ashamed".
Congresswoman Deb Haaland, one of the first Native American women to be elected to Congress, tweeted that the students showed "blatant hate, disrespect, and intolerance".
This Veteran put his life on the line for our country. The students’ display of blatant hate, disrespect, and intolerance is a signal of how common decency has decayed under this administration. Heartbreaking. https://t.co/NuPnYu9FP4— Congresswoman Deb Haaland (@RepDebHaaland) January 19, 2019
The Diocese of Covington and Covington Catholic High School later issued a statement offering "our deepest apologies to Mr Phillips".
"We condemn the actions of the Covington Catholic High School students towards Nathan Phillips specifically, and Native Americans in general," the statement reads.
It added that the incident was being investigated and "appropriate action" would be taken.
What did the footage show?
The teens are seen mocking Mr Phillips and his companion, as the two men sing the American Indian Movement song.
One boy, in particular, is captured staring at Mr Phillips while many other students are heard mockingly singing the song.
In separate footage later shared on social media, Mr Phillips is seen approaching the boys as he sings.
After the incident, Mr Phillips was quoted by US media as saying: "I heard them saying 'build that wall, build that wall'".
"This is indigenous land: you're not supposed to have walls here.
"We never had a prison; we always took care of our elders, took care of our children, always provided for them, taught them right from wrong.
"I wish I could... put that energy to making this country really, really great," Mr Phillips said.
Correction 25 January 2019: In subsequent US media interviews Mr Phillips said that while he had served in the Marine reserves, he was never deployed to Vietnam and so we have removed references describing him as a Vietnam veteran.
Update 29 January 2019: One of the students shown in the footage, Nick Sandmann, subsequently disputed Mr Phillips' account of the incident, as additional video footage from the day emerged. We reported these developments in a second article published the following day.