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DALLAS — Five officers were shot to death during protests against police in downtown Dallas, marking the deadliest single attack on U.S. law enforcement since September 11, 2001.

The gunfire started Thursday night as demonstrators marched against the shooting deaths of two men by police in Louisiana and Minnesota.

A total of 12 officers were shot.

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.

Two  protesters were also shot, including a woman who was shielding her children during the shooting.

Three suspects are in custody but not speaking with police.

Dallas Police Chief David Brown told reporters a fourth suspect, identified as 25-year-old Micah X Johnson of Mesquite,  was killed after hours of negotiations.

He said Johnson told them he was upset with police shootings and wanted to kill white people, especially white police officers.

Johnson eventually was killed by a bomb that authorities detonated, Brown said.

“We saw no other option but to use our bomb robot and place a device on its extension for it to detonate where the suspect was,” Brown said. “Other options would have exposed our officers to grave danger. The suspect is deceased as a result of detonating the bomb.”

Here’s what we know:

— A total of 12 police officers were shot by snipers during the protests, Dallas Police Chief David Brown 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, identified as 25-year-old Micah X Johnson of Mesquite, for hours at a parking garage in downtown.
— “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. Police say they detonated an explosive device near the man which killed him.
— K-9 units are sweeping the area for possible bombs.
— 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.
from his hotel balcony about 50 yards away. He described the gunman as carrying a weapon with a “pretty big magazine.”
— 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 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. The bangs got louder, and protesters realized it was gunfire.
“That went on for a while,” he said. Crowds ran into a parking garage, and 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.”