Dallas Police Chief David Brown says several officers testifying on the fatal shooting of five colleagues last month were overwhelmed when they returned to the scene and that some could need up to five years of treatment to recover from the trauma.

Brown told the city council Tuesday that more than 300 witnesses have been interviewed in the investigation into the shooting July 7 during a protest march in downtown Dallas.

The investigation includes having officers who worked that night return downtown to explain the timeline of events. Brown says some officers were overcome by emotion and had to postpone giving their accounts.

He says a four- or five-year treatment program is necessary to provide the necessary counseling for some officers.

The shooter, 25-year-old Micah Johnson, was killed by authorities.