This is just a supposition, but...
It's possible that campus police thought that the first 2 murders were isolated, that there was'nt gonna be a shooting spree, and that they're best chance to catch the shooter was to keep everyone on campus.
Because it happened on campus, you initially have 26,000 suspects.
And it takes time to narrow it down.
So, for what it's worth, until the next shootings, the campus PD was most likely thinking that the initial double homicide was the worst of it.That is, until the further shootings.