DALLAS — (CNN) Twelve officers were shot — five are dead, and seven are injured — in the attack in downtown Dallas, the Dallas mayor’s office said Friday. Two civilians also were injured, according to Scott Goldstein, spokesman for the mayor.

The ambush is the deadliest single attack on law enforcement since September 11, 2001.

Gunshots rang out Thursday night during demonstrations against the fatal shootings of two African-American men by police in Louisiana and Minnesota.

A total of 11 officers were shot, Dallas police said.

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.

Here’s what we know:

— A total of 11 police officers were shot by snipers during the protests, Dallas Police Chief David Brown said. An 11th officer was shot during an exchange of gunfire with a suspect, authorities said.

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

— 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.

— K-9 units are sweeping the area for possible bombs.

— “The suspect told our negotiators that the end is coming,” Brown said. The suspect at the garage 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. Officer Thompson married a fellow Transit Officer just two weeks ago.

— 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, who is in Warsaw, Poland, said his team is keeping him updated. “We still don’t know all the facts, we do know there’s been a vicious, calculated and despicable act on law enforcement,” Obama said. “I believe I speak for every American when I say we are horrified.”

Peaceful protest shattered

Witnesses said the protesters were marching peacefully when gunshots rang out. Crowds suddenly scattered in all directions.

“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 at first he thought it was fireworks. 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 said she 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.”

Police officers crouched behind vehicles while others approached the scene holding protective shields.

Officers have cleared the streets and are going around parking garages, looking for the shooter, Ben Ferguson, who lives in Dallas, told Lemon.

All rail and bus services have been suspended.

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 the convenience store in Baton Rouge, Louisiana, where Alton Sterling was fatally shot while police tackled him in a parking lot Tuesday.

They vented their rage at yet more killings of black men at the hands of police officers.

“We are targets!” LaRhonda Talley said in an impassioned speech in Minnesota about the danger of being black in America. “We made it across the (Atlantic). 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!”

Minnesota shooting

As has become the horrible norm, both killings were captured on video and posted online, spreading the outrage across the country at lightning speed.

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

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 evening shooting on Facebook.

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

In interviews throughout the day, Reynolds complained that other officers responding to the scene were more concerned about the officer who fired the shots than Castile.

Baton Rouge shooting

Sterling, 37, was killed Tuesday outside the Triple S Food Mart in Baton Rouge, where he regularly sold CDs and DVDs, earning the nickname “The CD man.”

A source close to the investigation told CNN that problems started when a homeless man approached Sterling on Tuesday and asked for money, becoming so persistent that Sterling showed him his gun, the source said.

“I told you to leave me alone,” Sterling told the man, according to the source.

The homeless man called 911 and police arrived at the store. Sterling was tackled and taken to the ground, the video shows, and during the scuffle he was shot several times by police.

A law enforcement source with knowledge of the investigation 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.

A prayer vigil was held at a church. A minister urged anybody who carried a weapon to take it out of the sanctuary and store it in their car.

In New York, about 1,000 people marched up Fifth Avenue toward Union Square chanting “Black lives matter!”

They didn’t have a permit, but police said they would allow the march to continue as long as it remained peaceful.

Tributes to slain men

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

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

Pop star Beyoncé posted a message on her website, urging action.

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

Most major cities reported a gathering of some sort. Little violence was reported, but many tears were shed.

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