A peaceful protest against nationwide officer-involved shootings took a sinister turn at about 9 p.m. Thursday night after snipers shot 11 police officers, killing five, according to Dallas Police Chief Brown.

DALLAS, TEXAS (CBSDFW.COM) – “Some officers were shot in the back,” said Brown, who added during a press conference that they were attacked “ambush-style.”

One suspect died of a self-inflicted gun shot wound at about 2:50am after an hours-long stand-off with police. Another suspect – an unidentified woman – was said to be in custody.

At about 3:30am Maj. Max Geron of the Dallas PD tweeted, “There are extensive sweeps of downtown for explosives underway. This will take quite a while.”

Early Friday morning Chief Brown sent the following statement via email before the fourth officer died.

Tonight it appears that two snipers shot ten police officers from elevated positions during the protest/rally. Three officers are deceased, two are in surgery and three are in critical condition. An intensive search for suspects is currently underway. No suspects are in custody at this time. We ask that any citizen with information regarding the shootings tonight call 214-671-3482.