Washington DC - Former President Donald Trump on Friday hit back against suggestions his wound from an assassination attempt was caused by "shrapnel," posting a letter from his former doctor saying it was almost certainly due to a bullet.

"There is absolutely no evidence that it was anything other than a bullet," wrote Ronny Jackson, now a Republican congressman from Texas, on Trump's social media platform Truth Social. Trump reposted the message.

Federal Bureau of Investigation (FBI) chief Christopher Wray told lawmakers on Wednesday that there was some doubt "whether or not it's a bullet or shrapnel that, you know, that hit his ear."

Trump was wounded on his right ear at a campaign rally on July 13 in Pennsylvania, surviving what the FBI has termed an assassination attempt when a gunman fired eight bullets at him during a speech.

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There has been no confirmation of the nature of Trump's wound from either medical, law enforcement, or government authorities, and Wray's comments were the first on-the-record details from a senior official on the matter.

Two rally attendees were seriously wounded in the attack, and a 50-year-old Pennsylvania firefighter was shot dead, according to officials.

The gunman was killed by a Secret Service sniper.

Since the shooting, Trump has made the attack a key part of his campaign pitch, telling a crowd in Michigan that he "took a bullet for democracy."

At the Republican National Convention where he was anointed the party's nominee for president, Trump said he had "God on my side" as he described the attack.

On Thursday, Trump denied Wray's comments and accused him of political partisanship.

"It was, unfortunately, a bullet that hit my ear, and hit it hard. There was no glass, there was no shrapnel," he said.

A New York Times investigation published Friday said "a detailed analysis of bullet trajectories, footage, photos and audio... strongly suggests Mr Trump was grazed by the first of eight bullets fired by the gunman."

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Trump's campaign has not released any medical reports or statements from his current physician, instead quoting Jackson – a former White House physician who is a staunch political ally of the ex-president.

Jackson has been the subject of controversy in the past, with a 2021 US Defense Department investigation finding he had "disparaged, belittled, bullied, and humiliated subordinates" while serving in the White House.

It also found that he had "made sexual and denigrating statements about one of his female medical subordinates," and raised concerns about his use of sleeping pills while on duty.

Jackson was demoted from his rank of rear admiral to captain by the US Navy following the investigation, media reported in March.