Officers reportedly delayed confronting the gunman despite knowing children were wounded inside a classroom.
School district police chief Pete Arredondo claimed that officers risked their lives without hesitation.
Nineteen children and two adults were killed in the 24 May shooting at Robb Elementary.
The police response has come under increasing scrutiny in the weeks since the shooting, in which the gunman was locked in two adjoining classrooms with students for more than an hour before police stormed in and killed the attacker.
Some parents have condemned the response as being chaotic and slow.