Eric Trump, son of US President Donald Trump, said he was spit at by an employee at a sophisticated cocktail bar in Chicago.
"It was a purely disgusting act by someone who clearly has emotional problems," Trump told Breitbart, a right-wing news site.
The employee was interviewed by intelligence agents and released later, according to local authorities.
The Bar confirmed that an "unfortunate incident" had occurred with Mr. Trump.
"We have not witnessed the incident and are just beginning to get the details," the aviary said in a statement. "One thing is certain: no customer should ever be spit on."
Mr. Trump told Breitbart that he refuses to file charges against the employee who had been given leave from the bar.
A Chicago police spokesman confirmed on Twitter that local officials had assisted the intelligence agency with a "law enforcement matter" Tuesday night, but did not elaborate.
The incident occurred at Aviary, a James Beard Award winning cocktail bar in Chicago's West Loop district.
According to local media, Mr. Trump was at the bar around 8:30 pm local time on Tuesday (01
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Nick Kokonas, co-owner of the Aviary, made headlines in January this year when he invited the Clemson Tigers, national college football champions , to "a real feast" in his restaurant Alinea, after the president had served fast food to the players in the White House.
President Trump, who hosted the team during the longest standstill of the US government, personally paid for the meals.
Mr. Trump, Executive Vice President of the Trump Organization, was in Chicago for a visit to the Trump Hotel in Chicago.
He told Brietbart that he believed the incident was politically motivated.
"For a party that preaches tolerance, this again shows that they have very little courtesy," said the third child of the president, indicating that the employee was a Democrat.
"It only highlights a disease and despair and the fact that we win."
The alleged spitting is part of a series of incidents in which members of the Trump team were harassed or confronted with political differences while eating out.
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Last June, activists beat up then-Interior Security Minister Kristjen Neilsen at a Mexican restaurant in Washington DC. Days later, former White House spokeswoman Sarah Sanders was asked to leave a restaurant in Lexington, Virginia, for working for the Trump administration.