[Breaking news update 12:40 a.m. ET]

The man whose picture has been circulated by the Dallas Police Department has turned himself in, the department tweeted. Police initially called the man a suspect, but now refer to him as a person of interest. Another alleged suspect is in custody, the tweet said. A suspicious package was discovered near that suspect's location. The package is being secured by a bomb squad, the tweet said.

[Breaking news update 12:19 a.m. ET]

A fourth officer has died following a protest in Dallas over shootings by police of black men in Louisiana and Minnesota, Dallas police tweeted.

[Breaking news update 12:13 a.m. ET]

The Dallas Police Department tweeted an image of a man they said was one of the suspects and asked the public for help in finding him. The photo is of an African-American man wearing a camouflage T-shirt and carrying what appears to be a rifle. Texas is an open carry state, which means it is legal for those with permits to openly carry weapons.

[Breaking news update 12:08 a.m. ET]

Eleven police officers have been shot in Dallas, according to city police Chief David Brown. Three officers have died: one DART officer and two Dallas police officers, Brown said.

[Breaking news update 12:05 a.m. ET]

Police have cornered a suspect in a commercial garage after the shootings of 11 police officers near the end of a protest in Dallas over shootings by police of black men in Louisiana and Minnesota, police Chief David Brown told reporters. The chief said at least two snipers in elevated positions fired "ambush style" on the officers. "Some (were) shot in the back." There also is a search for a possible bomb in the area, Brown said. "This is a very emotional time for our department and the law enforcement community across the country," Brown said. Officials asked the public's help in identifying suspects.

[Previous story posted at 11:58 p.m. ET]

Multiple police officers have been killed during a protest in Dallas over shootings by police of black men in Louisiana and Minnesota.

Three Dallas police officers were killed and eight others were wounded, Dallas Police Chief David Brown and the City of Dallas said in separate statements.

One Dallas Area Rapid Transit officer was fatally shot, the agency tweeted.

It's not clear if Brown included the DART officers in his tally.

Brown said two snipers shot the 10 from elevated positions during a protest. Two officers are in surgery and three are in critical condition. No suspects were in custody.

Three other DART officers were also shot. Their injuries are not considered life-threatening, DART said.

'Everyone was screaming'

The shooting happened as protests were underway about two blocks from Dealey Plaza. Video showed the crowd suddenly sprinting away.

CNN affiliate KTVT reported that two Dallas officers were shot. CNN could not immediately confirm that information and it's not clear if they were referring to the DART officers.

Witness Clarissa Myles said she was eating at McDonalds when the chaos began.

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

"I was walking next to the officer who was helping with the protest, all of a sudden I saw six to eight shots," one witness told the station. "It looked like two officers went down."

Another witness who was at the protest told the station he heard multiple gunshots.

"Whoever was shooting had an assault rifle — and I know guns. The shots were in rapid succession," the witness said.

Video showed numerous police officers crouching behind vehicles. Others approached a location holding protective shields.

"Our thoughts and prayers are with the Dallas law enforcement community and the Dallas Area Rapid Transit (DART) officers killed and injured this evening," Texas Gov. Greg Abbott said in a statement following the shooting. "In times like this we must remember -- and emphasize -- the importance of uniting as Americans."

Protests across the country

The shootings occurred as Americans across the nation vented their anger over the police killings of two black men in two days.

They chanted outside the governor's residence in St. Paul, Minnesota, miles from the spot where an officer killed Philando Castile in a car on Wednesday while a 4-year-old girl sat in the back seat.

Crowds milled in the streets outside the convenience store in Baton Rouge, Louisiana, where Alton Sterling was fatally shot while police grappled with him in a parking lot Tuesday.

Protesters briefly shut down the Dan Ryan Expressway in Chicago. In New York, 1,000 people marched down Fifth Avenue and a few scuffled with police officers.

They all came out to vent their rage at yet more slayings 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!"

Beyoncé pays tribute to slain men

The mighty also raised their voices.

President Obama talked about the shootings, saying "This is not just a black issue." Democrats came out as a group onto the steps of Congress to show their support for the victims.

Beyoncé, one of the most famous American pop stars, posted a message on her website, saying "It is up to us to take a stand and demand that they 'stop killing us.'" The singer provided a link for fans to contact their congressmen.

Virtually every major city reported a gathering of some sort. Little violence was reported, but many tears were shed.

Valerie Castile, mother of Philando Castile, told a crowd gathered outside J.J. Hill Montessori Magnet School in St. Paul, where Castile worked, that somebody "needed to police the police."

"It was my son today but it could be yours tomorrow," she said. "This has to cease. This has to stop, right now."

She also urged an economic boycott to bring attention to the killings of black men. "The only thing they know is money," she said.

Protesters in New York.

Captured on video

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

The shooting of Castile was remarkable -- and heartbreaking -- because it was live-streamed by his fiancee , who 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 Castile, along with her 4-year-old daughter.

"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. Later she says, "Oh God, please don't tell me my boyfriend is dead."

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.

A woman rings the doorbell at the gate of the governor's mansion in St. Paul, Minnesota.

'We need justice out here'

As news of Castile's killing spread, a crowd formed at Minnesota Gov. Mark Dayton's residence and grew throughout the day. Many ethnic groups were represented, including Aztec dancers who said they wanted to show the diversity of the community.

Among the protesters was Michael House, who said he grew up in the same neighborhood with Castile.

"I'm here because my longtime friend got killed by a police officer and we need justice out here," he said.

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.

Protesters block traffic and dance on cars near the Triple S Food Mart in Baton Rouge.

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. Tokens, 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. Video later showed police scuffling with a few protesters but it wasn't known if anybody was arrested.