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DALLAS – Shooters killed five officers during protests against police in downtown Dallas, marking the deadliest single attack on U.S. law enforcement since September 11, 2001.

Gunfire rang out Thursday night as demonstrators marched against the shooting deaths of two African-American men by police in Louisiana and Minnesota.

A total of 12 officers were shot, Dallas police said, along with two civilians. Dallas police negotiated and exchanged gunfire with a suspect for hours at a parking garage in downtown. That suspect is dead, a law enforcement official told CNN. The official did not say how the suspect died. Three people were in custody, police said.

Police swept for explosives after a suspect said bombs were planted all over the city. Dallas police Maj. Max Geron said police didn’t find any explosives during primary and secondary sweeps of the area.

It was the deadliest single attack on law enforcement since the 2001 terror attacks, when 72 officers died, according to the National Law Enforcement Officers Memorial Fund.

President Obama made a statement on the shootings from Warsaw, Poland, where he and other world leaders have gathered for a NATO summit. The president called the Dallas shootings a “vicious, calculated and despicable attack on law enforcement.”

“There’s no possible justification for these kinds of attacks or any attack on law enforcement,” he said. “Anybody involved will be held accountable.”

Here’s what we know:

— Some of the officers injured are undergoing surgery.

— Brown said it’s unclear how many suspects were involved, but three people are in custody.

— Dallas police have been negotiating and exchanging gunfire with a suspect for hours at a parking garage in downtown.

— “The suspect has told our negotiators that the end is coming,” Brown said. The suspect also told negotiators more officers are going to get hurt, and that bombs are planted all over downtown Dallas.

— Two of the shooters were snipers, who fired “ambush-style” from an “elevated position,” Brown said.

— Officers killed include one DART officer. DART, the Dallas Area Rapid Transit agency, operates buses and commuter rail in the city and surrounding suburbs.

— DART identified the officer killed as Brent Thompson, 43. He joined the transit agency in 2009, and was its first officer killed in the line of duty, DART tweeted.

— The Dallas Police Department circulated a photo of a man they said was a suspect in the shooting, but later called him a person of interest and said he turned himself in.

— Witness Ismael Dejesus said he filmed the shooter from his hotel balcony about 50 yards away. He described the gunman as wearing tactical pants and a tactical shirt. He had a weapon with a “pretty big magazine,” he said.

— “He got out of there, walked over to the pillar, put a magazine in and started firing,” he said. “It did look planned. He knew where to stand, he had ammo ready.”

— Retired FBI special agent Steve Moore said an attack of that magnitude required advance work.

— “This was an attack planned long before– waiting for an opportunity to go,” Moore said. “I think there was so much logistically, ammunition-wise. They may not have planned the location, they may not have planned the vantage point. But they had prepared for an attack before last night’s shooting is my guess.”

— President Barack Obama has been notified about the shooting, and a team is keeping him updated, the White House said.

Peaceful protest shattered

Witnesses said the protesters were marching peacefully when the gunfire started. Crowds scattered.

“In the midst of it, gunshots just started barreling out,” witness Michael Jackson told CNN’s Don Lemon. “I immediately started running the opposite way.”

G.J. McCarthy said he thought it was fireworks at first. It got louder, and protesters realized it was gunfire.

“That went on for a while,” he said. Crowds ran into a parking garage, then spilled out after word spread that there was a sniper nearby.

Clarissa Myles was eating at a McDonald’s nearby when peaceful protests suddenly turned chaotic.

“Everyone was screaming, people were running,” she said. “I saw at least probably 30 shots go off.”

Two killings in two days

The shootings occurred as Americans nationwide took to the streets to demand answers over the killings of two black men in two days. They wept, marched and chanted “Black Lives Matter!”

Crowds gathered outside Gov. Mark Dayton’s residence in St. Paul, Minnesota, miles from the spot where an officer killed Philando Castile in a car on Wednesday.

Hundreds of miles away, more protesters marched outside a convenience store in Baton Rouge, Louisiana, where Alton Sterling was fatally shot Tuesday while police tackled him in a parking lot.

“We are targets,” LaRhonda Talley said in an impassioned speech in Minnesota. “We made it across the transatlantic. We made it to freedom and you’re still killing us. You’re still hanging us from trees. You’re still killing us. Our lives matter! My son’s life matters. He matters to me … just like everybody’s son matters to their mama.”

Minnesota shooting

As has become a horrible norm, both killings were captured on video and posted online.

In Minnesota, the shooting of Castile was remarkable — and heartbreaking — because his fiancee live-streamed it.

As their 4-year-old sat in the backseat, she calmly narrated the action and showed viewers the dying man groaning and bleeding in the front seat.

Castile, a school food services worker, was shot in Falcon Heights, outside Minneapolis, when a police officer pulled him over because of a broken taillight, said his fiancee, Diamond Reynolds, who was in the car with him.

“He let the officer know that he had a firearm and he was reaching for his wallet and the officer just shot him in his arm,” Reynolds said as she broadcast the Wednesday shooting on Facebook.

“Oh God, please don’t tell me my boyfriend is dead,” she later says.

Reynolds said police responding to the scene were more concerned about the officer who fired the shots than Castile.

Baton Rouge shooting

Sterling, 37, was killed Tuesday at a convenience store in Baton Rouge, where he regularly sold CDs and DVDs.

Problems started when a homeless man approached Sterling on Tuesday and asked for money, becoming so persistent that Sterling showed him his gun, a source told CNN.

The homeless man called 911 and police arrived at the store. Police tackled Sterling to the ground, and shot him several times.

A law enforcement source told CNN that the officers pulled a gun from Sterling’s body at the scene. No further details were provided on the type of firearm.

The convenience store quickly became the site of protests. Flowers and signs piled up in a makeshift memorial. Protesters chanted “Hands up, don’t shoot,” the line made famous in the Michael Brown shooting in Ferguson, Missouri, about two years ago.

Tributes to slain men

As outrage grew over the shootings, Democrats came out as a group onto the steps of Congress to show their support for the victims.

President Barack Obama talked about the shootings, saying “This is not just a black issue.”

Celebrities urged action, and decried the killing of black men.

“It is up to us to take a stand and demand that they ‘stop killing us,'” pop star Beyonce said with a link for fans to contact their lawmakers.

Most major cities, including Chicago and New York, held protests demanding an end to shootings by police.