That was my thought, too. I can understand how they id'ed Hinrichs, but usually it takes awhile to go through friends or records to find out (a) there is a roommate, and (b) what his name is.
But maybe the authorities suspected he would commit suicide in a dramatic fashion. (Yes, that's sarcasm, in case anyone wonders.)
If they're very motivated, and choose to assign a bunch of people, it can take a short time.
They have the address. A bunch of cops show up and ask everybody there what they know. Meanwhile another bunch of cops go to the University office building, having called up relevant people and told them to be there with the keys. Go thru the records computer to see who has the same address and apartment # as Hinrichs.
Meanwhile, it would be hard for word to not get back to Cheema that they're looking for him. Cops and FBI probably decided to tap every line in the building (known to be mostly Muslim occupied) to see if anybody makes any calls to tip off Cheema that the cops are looking for him
Since the FBI is involved, they can rather quickly find out Cheema's cell number, and ask the cell company to ping the phone to find out what cell area its in. Once they know the area, they have equiptment that can ping the cell phone and triangulate on its position