None of the law enforcement personnel ventured into the building. Should they be blamed for that? Partially.
Could they have easily ventured into the building just minutes after the massacre had started, after the perimeter had been secured?
Yes, but I don't completely blame them for not doing it. They were receiving reports that made them overestimate the suspects' capabilities.
False reports of:
1. Up to 4+ shooters.
2. The shooters capable of booby-trapping bombs to backpacks, bodies, doors, etc.
3. A sniper on the roof.
4. Shooters have "grenades" (could be interpreted as more professional explosive devices)
5. Shooters have fully automatic weapons.
With reports like that, it sounded more like a job for the SWAT team and the SWAT team only. Even to the SWAT team, that was intimidating.
I don't doubt that if the police knew that they were dealing with 2 teenagers with low quality firearms, poorly made explosive devices, and very little shooting experience, they would have entered the school much earlier.