Five Dallas police officers were killed and at least six more injured in a coordinated sniper attack during an anti-police brutality protest Thursday, an explosion of violence that President Obama declared a vicious, calculated and despicable attack on law enforcement.

Three people were in custody and one suspect dead as local police and federal authorities raced to determine a motive and whether more people were involved in what was the nation’s deadliest day for police since 9/11. The suspects were not cooperating, officials said. Law enforcement veterans told Fox News the attack was clearly coordinated and showed an alarming degree of planning and sophistication.

“It’s a heartbreaking moment for the city of Dallas,” Mayor Mike Rawlings said. “I ask that everybody focus on one thing right now, and that is Dallas police officers, their families, those that are deceased [and] those that are in the hospital fighting for their lives.”

Dallas Police Chief David Brown said it appears multiple rifle-toting suspects were working together, “triangulating at different positions” as protesters marched through the streets of Dallas. The protest was one of several around the country, prompted by police shootings of black men in Louisiana and Minnesota.

Brown said late Thursday police did not know if all of the shooters and possible conspirators had been accounted for. One man killed himself after holing up in a parking garage, a woman was taken into custody near the garage and two men seen packing a camouflage bag into a Mercedes before speeding from the scene were apprehended and detained, the mayor said.

Brown said multiple shooters positioned themselves in two parking garages in downtown Dallas and “planned to injure and kill as many law enforcement officers as they could.”

A Dallas police source estimated to Fox News that at least 60 rounds were fired over a “large kill zone.” The source added that the shooting would have required considerable planning.

The suspect who killed himself had claimed that explosives had been set around the city, and much of downtown Dallas was locked down while police searched before determining there were no bombs. He died after first barricading himself in at the El Centro Community College parking garage, firing at police and warning the “the end is coming,” according to KDFW-TV.

Brown offered no possible motive or identities for the suspects, but a news conference was scheduled for Friday morning.

Obama, speaking from at a NATO summit in Poland, said America is horrified over the shootings, asked all Americans to pray for the fallen officers and their families and renewed his calls for more gun control.

Theres no possible justification for these kinds of attacks or any violence against law enforcement, Obama said, hours after a pre-attack speech in which he cited two racially charged police shootings earlier in the week and called for an end to bias in law enforcement.

One of the cops killed was identified as Dallas Area Rapid Transit Police Officer Brent Thompson, 43. He is the first DART officer to be killed in the line of duty. The others were picked off as the stood guard during the protest.

Thompson is the first officer to be killed in the line of duty since DART formed a police department in 1989, spokesman Morgan Lyons said.

“Our hearts are broken,” DART spokesperson Morgan Lyons said in a statement. “This is something that touches every part of our organization. We have received countless expressions of support and sympathy from around the world through the evening. We are grateful for every message.”

Three other DART officers were wounded, but they are expected to recover, Lyons said. As many as three city police officers were reportedly in critical condition.

Witness Carlos Harris told the Dallas Morning News the gunfire was “strategic. It was tap-tap-pause. Tap-tap-pause.”

Texas Gov. Greg Abbott released a statement saying he has directed the Texas Department of Public Safety director to offer “whatever assistance the City of Dallas needs at this time.”

“In times like this we must remember — and emphasize — the importance of uniting as Americans,” Abbott said.

The protesters had gathered after a Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child in a St. Paul suburb. The aftermath of the shooting was purportedly livestreamed in a widely shared Facebook video.

A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a cellphone video.

Other protests across the U.S. on Thursday were peaceful. In midtown Manhattan, protesters first gathered in Union Square Park. In Minnesota, where Castile was shot, hundreds of protesters marched in the rain from a vigil to the governor’s official residence. Protesters also marched in Atlanta, Chicago and Philadelphia.

Anti-police protests have roiled the nation in each of the last two summers following controversial police shootings, including the 2014 shooting of Michael Brown in Ferguson, Mo., and last April’s death of Fraddie Gray while in custody of Baltimore police.

A Department of Justice investigation cleared the police officer who shot Brown, and, of the six Baltimore police officers charged in Gray’s death, two have been acquitted, one’s case was declared a mistrial and three more face trial.

The attack made Thursday the deadliest day for law officers since Sept. 11, 2001, when 72 officers died, according to the National Law Enforcement Officers Memorial Fund.  

Fox News’ Bret Baier Casey Stegall and the Associated Press contributed to this report.