DALLAS (AP) — Flags have been lowered to half-staff and law enforcement badges from across the country were fastened to a tree outside of Dallas police headquarters as Saturday marked the two-year anniversary of a shooting attack that left five officers dead.

The date of the police ambush , July 7, 2016, is defined as the deadliest day for U.S. law enforcement since the Sept. 11 attacks. An Army veteran fatally shot four Dallas police officers and one transit officer before authorities killed him using a robot-delivered bomb.

The Dallas Morning News reports nearly 200 people gathered for a ceremony Friday to honor the fallen officers, along with another killed in the line of duty earlier this year. Police Chief U. Renee Hall says department personnel faced adversity but became the "beacon of courage and public service" to law enforcement officers.