Update 2:15 a.m.: A fifth officer wounded in Thursday night's sniper attack has died, according to a statement on the police Twitter account.
Update: 12:45 a.m.
Authorities are currently in negotiations with a suspect in downtown Dallas, and the suspect is shooting at police from the El Centro garage. Chief David Brown said that plans are being drawn up to end the siege. "As soon as I'm done here I'll be presented with those plans," Brown said. The suspect "told our negotiators that the end is coming and that he is going to hurt and kill more of us and that there are bombs all over the garage and downtown. So we are being very careful with our tactics."
Police have rounded up several suspects who left the scene, and are questioning them. "We still don't have complete comfort level that we have all the suspects," Brown said. Those suspects in custody are not cooperating, Brown said.
Mayor Mike Rawlings said that large sections of downtown Dallas will be sealed off as a crime scene. The closures will be listed on dallascitynews.net.
"It is a heartbreaking morning," Rawlings said.
Update 11:25 p.m.: Eleven officers from the Dallas Police Department and Dallas Area Rapid Transit were wounded,
two three DPD and one DART officer fatally, DPD Chief David Brown said at a brief City Hall news conference.
At least two shooters from separate elevated positions above the demonstration site opened fire, Brown said.
One of the suspects is cornered in the El Centro College parking garage, and officers are attempting to negotiate his surrender now.
Brown released this photo of a man he described as "a person of interest" and asked anyone with information about the man to call 214-671-3482. Update: (The man in the photo, Mark Hughes, was not involved with the shooting. He talked to DPD and was released shortly after his picture was circulated. After shooting began at the rally, Hughes' brother told reporters, Hughes turned over his legally-carried gun and went home.)
Update 11:15 p.m.: Reporter Stephen Young's parents live downtown above the location of tonight's deadly police shooting and captured the sounds of rapid, sustained gunfire in this video:
Update 10:50 p.m.: A statement from Dallas police Chief David Brown has been issued.
We will provide more information once it is available. Please keep us in your thoughts and prayers tonight.
Update 10:10 p.m.: The scene was chaotic when the shots broke out shortly before 9 p.m. with some bystanders running for cover while others began to shoot videos of the action to post online.
A bystander told the Observer he heard "50 to 75 shots," though estimates of the number of shots varied wildly.
Earlier in the evening, several hundred people gathered peacefully to protest the fatal shooting by two police officers of Alton Sterling outside a Baton Rouge convenience store early Tuesday. A video of the shooting posted online showed Sterling being wrestled to the ground and shot in the chest repeatedly.
Wednesday evening, Philando Castile, 32, was shot dead during a traffic stop for a broken taillight in a suburb of St. Paul, Minnesota. His girlfriend broadcast his final moments live on Facebook.
Update 10:01 p.m.: Jason Whitely of WFAA is reporting that one Dallas police officer and DART officer were wounded by the gunman.
A large swath of downtown has been blocked off, and police have cleared the park as they confront the shooter. A police officer at the scene said the gunman was carrying a rifle and was in an alley across the street from Belo Garden.
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Original story: Dallas police have cordoned Main Street and the area surrounding Belo Garden downtown following reports of multiple gunshots after a demonstration to protest recent police shootings of black men in Baton Rouge and Minnesota. Unconfirmed reports from bystanders say two police officers may have been wounded. At 9:40 p.m., police ordered reporters and bystanders to move back from the Main Street at Belo Garden because the shooter was seen moving in an alley, carrying a rifle.
Several witnesses posted videos of the scene during the shooting on Facebook.
Stephen Young is on the scene, and we'll have more as information comes available.