This is a developing story. Last updated 6:18 a.m. ET.
Snipers shot and killed five Dallas law enforcement officers and injured another six at the end of a rally in downtown Dallas, where hundreds were protesting police shootings that happened in other parts of the country earlier this week.
Four of the officers worked for Dallas Police; the fifth was identified as 43-year-old transit officer Brent Thompson, of Dallas Area Rapid Transit, or DART.
Dallas Police Chief David Brown described what he believes is a coordinated attack that intentionally targeted police officers, with at least four people — three men and one woman — involved. Some officers were shot in the back, Brown said.
"We still don't know all the facts. What we know is there has been a vicious, calculated and despicable attack against law enforcement," President Obama said, while traveling in Warsaw. "Police were on duty doing their jobs, keeping people safe during peaceful protests."
A wide swath of the city will be blocked off throughout the morning and perhaps for the rest of the day Friday as police investigate the scene.
Three suspects are in police custody. One of the suspects was surrounded by officers at a parking garage in a nearly five-hour-long standoff. Dallas Police tell NPR member station KERA that the suspect is now dead. The circumstances of how he died are unclear.
The suspect involved in the standoff "told our negotiators that the end is coming," that he's "going to hurt and kill more [officers]," and that there were bombs planted all over the garage and downtown.
A suspicious package was found near the garage, and a bomb squad swept several Dallas buildings for potential explosives, a sweep that concluded just before dawn. Dallas Police say FBI and Homeland Security officers are also on the scene.
Witnesses described several bursts of gunfire and a chaotic and terrifying scene on Main Street in downtown Dallas, where police searched and eventually surrounded the suspect at the parking garage. As a precaution, the campus of El Centro College in the heart of downtown is on lockdown.
The mayor asked that others who work downtown stay away from the area on Friday morning.
"It is a heartbreaking moment for the city of Dallas," Dallas Mayor Mike Rawlings said. "I ask everybody to focus on one thing right now, and that is Dallas police officers, their families, those that are deceased, those that are in the hospital fighting for their lives."
The shooting occurred just before 9 p.m. local time, during a rally and march in response to shootings earlier this week of black men by police officers in Baton Rouge, La., and suburban St. Paul, Minn. There were similar protests taking place all over the country. Participants describe a peaceful protest for nearly an hour, until the gunfire broke out.
A person of interest whose photo was distributed by police earlier in the night turned himself in and is being questioned, the department said. But the interviews haven't yielded much, so far, according to Brown. "We just are not getting the cooperation we'd like, to know that answer of why, the motivation, who they are," he said.
Javier Giribet-Vargas, an intern with Dallas member station KERA, was covering the protests when the shootings happened.
" 'I don't know what happened, but by the Bank of America building, there was a shooting; I ran, I dropped my phone.'
"Dozens of Dallas Police cars arrived immediately. Officers were out in force, wearing helmets, shields and holding assault weapons.
" 'I was taking cover and I could hear on the police radio that there was an injured person who was shot in the chest and that the suspects could have been in a black SUV,' Giribet-Vargas says. 'The police were saying through the radio that the shots might have come from the parking garage of one of the buildings.' "
The organizer of the protest, Dominique R. Alexander of the Next Generation Action Network, released a statement saying the organization "does not condone violence against any human being, and we condemn anyone who wants to commit violence."
In a statement, Texas Gov. Greg Abbott said, "In times like this we must remember and emphasize the importance of uniting as Americans."
(Editor's note: The following video includes images and language some may find disturbing.)
This is a rapidly developing story; we'll provide updates as they become available, including corrections should any of the above information prove to be incorrect.