Trump went on to blame President Obama for a recent increase in homicide.

"Decades of progress made in bringing down crime are now being reversed by this administration’s rollback of criminal enforcement," Trump said. "Homicides last year increased by 17 percent in America’s 50 largest cities. That’s the largest increase in 25 years."

Trump was apparently citing data compiled by The Washington Post on killings in major U.S. cities. The data, collected from local police agencies, shows that the number of homicides increased 17 percent. After years of steady improvements in public safety, the data from last year represented the greatest increase in a quarter century.

The increase is profoundly concerning to law enforcement officials and criminologists. Above all, the numbers represent the unimaginable tragedy of lethal violence experienced by the families and friends of more than 5,300 victims in these cities over the course of last year.

Fortunately, despite the recent increase, the rate of homicide in the 50 largest cities remains moderate relative to the recent past. The level of violence last year, about 10.8 homicides per 100,000 people, is less than the rate as of 2008, when there were 11.8 homicides per 100,000 people in this group of cities.

Experts on crime still do not have a good explanation for the increase last year, and theories range widely. For example, one conservative commentator recently pointed to the arrest of Joaquín “El Chapo” Guzmán, the notorious Mexican drug kingpin, in 2014. El Chapo's arrest, according to this theory, created a vacuum of power in the black market, setting off conflict among the cartels south of the border that reached north into American cities.

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Another possible explanation is that media coverage of recent police shootings of unarmed black men has damaged the reputation of police agencies in the black communities where much of the recent violence has been concentrated. On this theory, witnesses and informants have become unwilling to cooperate with detectives in investigations, making it more difficult for them to apprehend dangerous criminals.

This theory has some support in recent public polling on the police, which shows a marked decline in confidence in law enforcement among people of color and especially African Americans.

Other commentators have argued that the recent shootings have made police more hesitant to do their duties. According to this theory, officers are worried that something could go wrong and that they could become the focus of a national controversy. At this point, however, the evidence for this view is limited. Arrest rates in most cities have remained stable, suggesting that police are continuing to enforce the law amid all the turmoil.

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It was not immediately clear why Trump attributed the increase to the Obama administration. Policing is mainly the responsibility of local departments, and there have not been any significant changes in federal policy that might explain the recent violence.

Rhetorically, while Obama has acknowledged shortcomings in law enforcement with respect to racial discrimination, he has always spoken favorably of the police in general. In his public remarks on the issue, he has argued that most officers do their jobs well, and that historically, black neighborhoods would have benefited from more law enforcement. His administration has even called for a substantial increase in the number of cops on the street.

Trump has previously suggested that Obama is insufficiently supportive of the police. Asked to explain what he meant in an interview on "Fox & Friends" on Monday, however, the New York businessman declined to elaborate.