The review released Thursday was requested by Newtown's police chief. The panel visited Newtown, interviewed officers and studied documentation including 911 audio, police radio and video.
Adam Lanza killed 20 children and six women inside the school Dec. 14. He killed his mother at their home before driving to the school.
The panel said dispatchers alerted officers during the first 911 call, and they began arriving in less than three minutes. Officers entered the locked school about six minutes later, after determining someone seen outside wasn't another shooter.
It said officers "navigated the inevitable chaos" but were unable to intervene before Lanza committed suicide about a minute and 10 seconds after the first officer arrived.