Agents from the National Transportation Safety Board spent the morning inspecting the damaged helicopters, which crashed about 4 p.m. Saturday at the Pasadena Police Department's helipad in Altadena.
Six people were injured - five officers and one civilian observer. All were taken to area hospitals to be treated for minor injuries.
Officials say the helicopters' rotary blades may have touched during a maneuver. One helicopter was taking off while the other was landing at the time of the incident.
The NTSB has not confirmed what may have caused the accident. They said they plan to watch any available surveillance video and question the Pasadena police officers involved in the incident.
Pasadena Police Chief Phillip Sanchez said his department is cooperating fully.
"There are some lessons learned," Sanchez said. "So, we're going to look at this. I think that's the value of the NTSB."
Each Bell OH-58 helicopter was extensively damaged. One helicopter was on routine assignment to calls for service and the other was assigned to the USC-UCLA game at the Rose Bowl. Each helicopter had a pilot and an observer. Two of the injured were not on board.
The helicopters monitor not only Pasadena, but also Altadena, Glendale and Monrovia. Some residents were concerned that the damaged helicopters could affect police patrol coverage. Authorities say there's nothing to worry about.
"Having two helicopters down is a major problem for us," said Lt. Phlunte Riddle with Pasadena police, "but because we have six, four of which are still completely operational, we will be able to patrol our area as well as provide our mutual aid service to other areas in our partnerships and our taskforce."