Police officers at the scene of the Dallas shooting, in which five officers were killed. Photo: Twitter/IsraelHatzolah.

A gunman, possibly with an accomplice, opened fire on police officers in downtown Dallas late Thursday during a protest over recent police-involved shootings.

Five law enforcement officials were killed in the ambush, according to Dallas police. The attack marks the deadliest attack on law enforcement since Sept. 11, 2001, according to the National Law Enforcement Officers Memorial Fund.