DALLAS — Downtown Dallas descended into chaos on Thursday when snipers killed at least five police officers and injured six others in the deadliest day for U.S. law enforcement since 9/11. <br /> <br />Two unidentified gunmen opened fire on police from an elevated position as anti-violence protesters demonstrated in the city, police said. <br /> <br />The demonstration was held in response to two fatal shootings this week of black men by police officers in Minnesota and Louisiana. <br /> <br />One of the police officers killed was named as Brent Thompson, 43. He is the first Dallas Area Rapid Transit (DART) officer to be killed on duty, KDFW reported. <br /> <br />Witnesses told CNN they saw a gunman with a rifle and wearing tactical gear exchanging fire with police officers at street level outside the El Centro College. Amateur video footage appeared to show the suspect ambushing a man believed to be a police officer and shooting him dead. <br /> <br />One of the suspects was cornered by police in the parking garage and shot himself dead after a standoff, KDFW reported, citing police sources. <br /> <br />Police were inspecting a suspicious package found near the alleged sniper and are investigating his claims that bombs had been left in the garage and throughout the city, according to KDFW. <br /> <br />Police said three suspects were in custody, including a woman arrested close to the parking garage and two men, who were held after they were seen leaving the scene at high speed in a car. Police retrieved two camouflage bags from the vehicle. <br /> <br />The attack is the deadliest on U.S. law enforcement since 9/11, when 71 police officers were killed in New York and one died on United Airlines flight 93. <br /> <br />Investigators were trying establish a motive for the attack.
