Photo: AP

Authorities say they believe the attack in Dallas that killed five police officers and injured six more was carried out by at least four snipers who positioned themselves in “triangulated locations” on rooftops near the end of a Black Lives Matter parade route Thursday.

There were about 800 protestors and 100 police officers in the crowd when the shooting began around 8:45 p.m., the New York Times reports. Dallas Police Chief David O. Brown says police believe there were at least four snipers positioned above the end of the parade route, specifically targeting law enforcement officers.

“Some [officers] were shot in the back,” Brown said at a press conference Thursday night. “We believe that these suspects were positioning themselves in a way to triangulate on these officers.”

“[They were] working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going,” Brown said. “[They] planned to injure and kill as many law enforcement officers as they could.”

One of the officers killed has been identified by the AP as 43-year-old Dallas Area Rapid Transit officer Brent Thompson, a seven-year veteran of the force who became the first officer killed in the line of duty since the DART police force was formed in 1989. CNN reports he was a newlywed who married a fellow officer two weeks ago.

The other four Dallas Police officers who were killed have not yet been identified.

Brown told reporters at least one civilian was also injured in the shooting. Shetamia Taylor, 37, was reportedly shielding her four sons from the gunfire when she was struck in the calf by a bullet. Her sister tells the AP she underwent surgery for the wound early Friday morning.

Police officers—aided by the FBI, the Department of Homeland Security, and the Bureau of Alcohol, Tobacco, Firearms and Explosives—eventually took three suspects into custody and cornered a fourth suspect inside a parking garage early this morning. That suspect reportedly died after prolonged negotiations that included the exchange of gunfire from what’s been reported as a self-inflicted gunshot, though police have not yet confirmed his death. During the standoff, he allegedly warned police “that the end is coming and he’s going to hurt and kill more of us, meaning law enforcement, and that there are bombs all over the place in this garage and downtown,” Brown said.

At one point, federal authorities reportedly stopped commercial air traffic over the city to allow police helicopters to sweep the area.

Primary and secondary bomb sweeps of the city concluded around 3 a.m. yielding no evidence of explosives, Dallas Police Department major Max Geron said on Twitter early Friday morning.

At a press conference Thursday night, police initially identified a man photographed on the parade route with a rifle slung across his back as a “suspect,” who was later downgraded to a “person of interest.” His photograph was released by Dallas police and broadcast by most networks before it became clear he was Mark Hughes, the brother of one of the rally organizers, who apparently turned his gun over to a police officer as soon as the attack began.

The suspects currently in custody reportedly include two people who “were seen with a camouflage bag getting into a black Mercedes” and driving away at a high speed, and a woman, who was arrested near the garage.

On Thursday, Chief Brown indicated the suspects were not giving police much to go on and said authorities are not confident they have everyone involved in custody.

“We just are not getting the cooperation we’d like, to know that answer of why, the motivation, who they are,” he said.

The suspects, Brown said, appeared to have some familiarity with the parade route.

“How would you know to post up there?” he told the New York Times. “So we’re leaving every motive on the table of how this happened and why this happened... We have yet to determine whether or not there was some complicity with the planning of this, but we will be pursuing that.”

Speaking from Poland early Friday morning, where he is meeting with NATO officials, President Obama expressed horror over the attack, saying “there is no possible justification for these kinds of attacks, or any violence against law enforcement.”

“We still don’t know all of the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement,” he said. “We are horrified over these events, and we stand united with the people and the police department in Dallas.”