DALLAS -- Four officers were killed in downtown Dallas as crowds protested the fatal shootings of two African-American men by police in Louisiana and Minnesota, authorities said.
A total of 10 police officers were shot during the protests, Dallas Police Chief David Brown said. An 11th was shot during an exchange of gunfire with one of the suspects, and the officer was transported to the hospital, authorities said.
During a news conference after the shooting, Brown said there could be more than two shooters. He said police had cornered a suspect in a commercial garage.
Later in evening, Dallas police said two were questioned following a car chase:
— Dallas Police Depart (@DallasPD) July 8, 2016
"These suspects have threatened to plant a bomb in the downtown area," he said.
"We do know at least two snipers from elevated position as the protest rally march ended, began firing on officers ambush-style."
Officers killed included one DART officer and two Dallas police officers.
DART, the Dallas Area Rapid Transit agency, operates buses and commuter rail in the city and surrounding suburbs.
Report from 10 p.m.:
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. Then 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 started spreading that there was a sniper nearby.
Clarissa Myles said she was eating at a McDonalds 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!"
Protesters in other major cities nationwide marched peacefully, including in Chicago, and New York.
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.
Virtually every major city reported a gathering of some sort. Little violence was reported, but many tears were shed.