Four police officers were killed, at least seven were wounded and a civilian was also hurt late Thursday when two snipers opened fire during a protest in Dallas against police racial bias, authorities said.City Police Chief David Brown said two gunmen shot at ‘police officers from elevated positions during the protest/rally’, sparking an intense search for at least two suspects, adding that they had threatened to plant a bomb.The Dallas Police Department tweeted a photograph of a man dressed in a camouflage shirt with an assault rifle slung over his shoulder.‘This is one of our suspects. Please help us find him!’ the police department tweeted.The suspects ‘have threatened to plant a bomb in the downtown area,’ Brown told reporters.The protests were part of demonstrations taking part in several cities against the fatal police shootings of African American men in Minnesota and Louisiana this week.Brown initially said two of the officers were in surgery and three were in critical condition. The condition of the other officers was unclear.An earlier toll had 10 officers shot, including three dead. Brown told reporters a civilian was also wounded.‘An intensive search for suspects is currently under way. No suspects are in custody at this time,’ Brown said in a statement.Brown said police had ‘cornered’ one of the suspects, and warned that there may be more than two suspects.Dallas police has reached out to federal law enforcement agencies for help - the Federal Bureau of Investigation and Bureau of Alcohol, Tobacco, Firearms and Explosives.Several hundred people attended the Dallas rally, which ended just before the shots rang out at about 9:00pm (0200 GMT Friday), reports said.Witnesses reported hearing dozens of gunshots that sounded like semi-automatic rifle fire. ‘The shots were coming from the roof,’ a protester told KTVT television, a CBS affiliate.Video posted online showed protesters marching when the shots rang out, scattering the crowd.Police warned protesters to flee the area, saying the scene remained an active-shooter situation.SWAT teams were deployed to the scene, reports said.