Trump told reporters Friday evening that he didn't know the racially-charged history behind the phrase "when the looting starts, the shooting starts."
He tweeted the phrase Friday morning in reference to the clashes between protesters and police in Minneapolis following George Floyd's death.
It dates back to the civil rights era and is known to have been invoked by a white police chief cracking down on protests and a segregationist politician.
In 1967, Miami police Chief Walter Headley used the phrase "when the looting starts, the shooting starts," invoking angry reactions from civil rights leaders, according to a news report at the time.
"He had a long history of bigotry against the black community," said professor Clarence Lusane of Howard University.
According to Lusane, Headley may have borrowed the phrase from Eugene "Bull" Connor, who had been the notorious public safety commissioner in Birmingham, Alabama.
Connor was a segregationist who directed the use of police dogs and fire hoses against black demonstrators.
Segregationist presidential candidate George Wallace also used the phrase during the 1968 campaign.
"So often Trump has engaged in dog whistles," Lusane said. "But he also engages in blaring trumpets. And this is a pretty clear and very loud message that the response should not be let's try to address the justice issues that are involved here but let's be hard-line."