SWAT Sergeant Relives Horror of Columbine Massacre — Case I Can’t Forget

If you read the SWAT sections of the Jeffco/CNN CD, it's apparent they feared trip bombs as well as time bombs. Hence they tried the fire truck at the west entrance, and eventually used the teachers lounge window rather than use any doors.

The diversion was likely two bombs, one a time bomb, and the other a trip bomb to kill those responding to the time bomb. Again, the CNN CD says as much. Eric seems to say as much in his final entry about more frags on the scoreboard.