A man is alleged to have slammed a boy’s head to the ground at a county fair because the 13-year-old kept his hat on during the US national anthem.

Mineral County sheriff Mike Boone said witnesses identified the suspect as 39-year-old Curt James Brockway, who was apprehended at the fairgrounds and charged with assault on a minor.

The sheriff’s office declined to provide additional information on the alleged assault. But Taylor Henneck, who attended the event, told local news outlets she overheard the attack just as the national anthem began to play.

The woman said she heard a “pop” and turned to see the boy writhing on the ground.

“He was bleeding out of his ears, seizing on the ground, just not coherent,” she told The Missoulian.

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As startled spectators closed on Mr Brockway, Ms Hennick said he offered a simple defence for his actions.

“He said [the boy] was disrespecting the national anthem so he had every right to do that,” she added.

Police said the child was rushed to a hospital on Saturday then flown to Sacred Heart Children’s Hospital in Spokane, Washington.

The boy suffered temporal skull fractures in the incident, his mother told the KPAX new channel, adding that her son ears bled for six hours after the alleged assault.

He was later released from the hospital and was recovering at home, she said.

“It’s just a lot of pain in my head. I don’t remember anything – the rodeo – the helicopter – nothing,” the 13-year-old said in an interview with the station.

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The silent protests were largely condemned by many on the political right, including US president Donald Trump – who in September 2017 encouraged teams’ owners to fire players who knelt during the anthem.

Mr Brockway was charged with assault with a weapon in 2010, resulting in a 10-year probation sentence.

His arraignment is set for 14 August.