[WhoisAlanGreenspan?]

I’ve seen many references to so many minutes passing, but have not seen in what span of minutes the shooting took place, and where the policemen were during that time.

At 11:33 a.m. on May 24, he walked into Robb Elementary’s northwest entrance and headed south toward the two classrooms on the left side, randomly firing shots from his rifle in the hallway.
The gunman fired shots at least three more times — at 11:40 a.m., 11:44 a.m., and 12:21 p.m. — but officers held their positions.

From this link

I believe police were waiting at both ends of the hallway.

[Will88]

The gunman fired shots at least three more times — at 11:40 a.m., 11:44 a.m., and 12:21 p.m. — but officers held their positions.

The more that's learned, the worse it looks. Hard to believe policemen would stand around outside a classroom filled with kids knowing a gunman was firing shots as they waited.