DALLAS — Gunmen shot and killed five police officers and wounded seven others during a protest over fatal police shootings of black men in other states, authorities said. It appeared to be the deadliest day for U.S. law enforcement since the 2001 terrorist attacks.

Thursday’s bloodshed, which unfolded just a few blocks from where President John F. Kennedy was slain in 1963, also evoked the trauma of the nation’s tumultuous civil rights era.

Police Chief David Brown blamed “snipers” and said three suspects were in custody. Mayor Mike Rawlings said a fourth was slain by police in a downtown parking garage where he had exchanged gunfire with authorities.

“We don’t exactly know the last moments of his death, but explosives did blast him out,” Rawlings told The Associated Press. Police said the man had told negotiators he intended to hurt more law enforcement officials.

Police did not identify any of the suspects. The police chief said the dead suspect had declared before his death that he was upset about recent shootings and wanted to kill whites.

The shooting began about 8:45 p.m. Thursday while hundreds of people were gathered to protest the week’s fatal police shootings in Baton Rouge, La., and suburban St. Paul, Minn. Brown told reporters the snipers fired “ambush style” on the officers. Two civilians were also wounded, Rawlings said.

Brown said it appeared the shooters “planned to injure and kill as many officers as they could.” Video from the scene showed protesters marching along a downtown street about half a mile from City Hall when shots erupted and the crowd scattered, seeking cover.

“I think the biggest thing that we’ve had something like this is when JFK died,” resident Jalisa Jackson downtown said early today as she struggled to fathom the still-unsettled situation. Officers crouched beside vehicles, SWAT team armored vehicles arrived and a helicopter hovered overhead.

Demonstrations were held in several other U.S. cities Thursday night to protest the police killings of two more black men: A Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child, the shooting’s aftermath 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.

Thursday’s shootings occurred in area of hotels, restaurants, businesses and some residential apartments only a few blocks from Dealey Plaza, the landmark made famous by the Kennedy assassination.

The scene was chaotic, with officers with automatic rifles on the street corners.

Brown said police don’t have a motivation for the attacks or any information on the suspects. He said they “triangulated” in the downtown area where the protesters were marching and had “some knowledge of the route” they would take.

Video posted on social media appeared to show a gunman at ground level exchanging fire with a police officer who was then felled.

Authorities have not determined whether any protesters were involved with the attack and were not certain early today that all suspects had been located, Brown said.

Rawlings said authorities would likely ask some people to stay away from downtown Dallas today.

Early this morning, dozens of officers filled the corridor of the emergency room at Baylor Medical Center, where other wounded officers were taken. The mayor and police chief were seen arriving there.

Four of the officers who were killed were with the Dallas Police Department, a spokesman said. One was a Dallas Area Rapid Transit officer. The agency said in a statement that 43-year-old officer Brent Thompson was the first officer killed in the line of duty since the agency formed a police department in 1989.