Hundreds of federal agents already have been sent to Kansas City, Missouri, to help quell a record rise in violence after the shooting death of a young boy there.
Sending federal agents to help localities is not uncommon.
Mr Barr announced a similar surge effort in December for seven cities that had seen spiking violence.
Usually, the Justice Department sends agents under its own umbrella, like agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives or the Drug Enforcement Agency.
But this surge effort will include at least 100 Department of Homeland Security Investigations officers working in the region who generally conduct drug trafficking and child exploitation investigations.
DHS officers have already been dispatched to Portland, Oregon, and other localities to protect federal property and monuments as Mr Trump has lambasted efforts by protesters to knock down Confederate statutes.
Mr Trump has linked the growing violence in the streets with protests over racial injustice, though criminal justice experts say the spike defies easy explanation, pointing to the unprecedented moment the country is living through, a pandemic that has killed more than 140,000 Americans, historic unemployment, stay-at-home orders, a mass reckoning over race and police brutality, intense stress and even the weather.
And compared with other years, crime is down overall.
Local authorities have complained that the surges in federal agents have only exacerbated tensions on the streets.
The decision to dispatch federal agents to American cities is playing out at a hyper-politicised moment when Mr Trump is trying to show he is a “law-and-order” president and painting Democratic-led cities as out of control.
With less than four months to go before election day, Mr Trump has been serving up dire warnings that the violence would worsen if his Democratic rival Joe Biden is elected in November, as he tries to win over voters who could be swayed by that message.
Under no circumstances will I allow Donald Trump’s troops to come to Chicago and terrorize our residents.
— Mayor Lori E. Lightfoot (@chicagosmayor) July 21, 2020
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But civil unrest in Portland only escalated after federal agents there were accused of whisking people away in unmarked cars without probable case.
The spike in crime has hit hard in some cities with resources already stretched thin from the pandemic.
But the move to send in federal forces was initially rejected by local leaders.
Chicago mayor Lori Lightfoot later said she and other local officials had spoken with federal authorities and come to an understanding.
“I’ve been very clear that we welcome actual partnership,” the Democratic mayor said Tuesday after speaking with federal officials.
“But we do not welcome dictatorship.
“We do not welcome authoritarianism, and we do not welcome unconstitutional arrest and detainment of our residents.
“That is something I will not tolerate.”
Our local police departments do not need weapons of war. That's why I am supporting an amendment to the Defense Bill to end the transfer of tear gas and other military-grade equipment from the Pentagon to police.
We need to #DemilitarizethePolice.— Martin Heinrich (@MartinHeinrich) July 21, 2020
In New Mexico, meanwhile, Democratic elected officials were cautioning Mr Trump against any possible plans to send federal agents to the state, with US Senator Martin Heinrich calling on Bernalillo County Sheriff Manny Gonzales, who was at the White House on Wednesday, to resign.
“Instead of collaborating with the Albuquerque Police Department, the sheriff is inviting the President’s stormtroopers into Albuquerque,” the Democratic senator said in a statement.
But federal gun crimes generally carry much stiffer penalties than state crimes and larger-scale federal investigations that can cross state lines tend to make a big impact.
The Justice Department will reimburse Chicago 3.5 million US dollars for local law enforcement’s work on the federal task force.
Through a separate federal fund, Chicago received 9.3 million US dollars to hire 75 new officers.
Two dozen agents will be sent to Albuquerque, and the administration made available 1.5 million US dollars in funding for the Bernalillo County Sheriff’s Department for five new deputies and 9.4 million US dollars for 40 new Albuquerque officers.
In Kansas City, the top federal prosecutor said any agents involved in an operation to reduce violent crime in the area will be clearly identifiable when making arrests, unlike what has been seen in Portland.
“These agents won’t be patrolling the streets,” US attorney Timothy Garrison said.
“They won’t replace or usurp the authority of local officers.”
Operation Legend, named after four-year-old LeGend Taliferro, who was fatally shot while sleeping in a Kansas City apartment late last month, was announced on July 8.
The first arrest came earlier this week.
Mr Garrison has said that the additional 225 federal agents from the FBI, DEA, ATF and the US Marshals Service join 400 agents already working and living in the Kansas City area.
The Trump administration is facing growing resistance in Portland.
Multiple lawsuits have been filed questioning the federal government’s authority to use broad policing powers in cities.
One suit filed on Tuesday says federal agents are violating protesters’ 10th Amendment rights by engaging in police activities designated to local and state governments.
Oregon’s attorney general sued last week, asking a judge to block federal agents’ actions.
The state argued that masked agents had arrested people on the streets without probable cause and far from the US courthouse that has become a target of vandalism.
Federal authorities, however, said state and local officials had been unwilling to work with them to stop the vandalism and violence against federal officers and the US courthouse.
