Four Dallas police officers and one Dallas Area Rapid Transit officer have been killed in what the Dallas Morning News called a “coordinated attack during [a] demonstration against recent shootings of black men by police in Louisiana and Minnesota.” Altogether, 11 officers and one bystander were shot, in what was the deadliest attack for U.S. law enforcement since September 11, 2001. Two snipers are said to have carried out the assault. One suspect is in custody, and one is dead.

The attack comes at a fraught time in police-community relations. The nation was already reeling this week after two separate incidents, one in Louisiana, one in Minnesota, where black suspects were shot dead by police officers.