The culprit, I assume.

I just shrugged, wedged the storeroom open with a door stopper

and took the whiteboard inside by myself.

I wasn't really paying attention, but I could tell from the sound that the boiler room door was also open.

The boiler room is really noisy, after all.

There's a window in the boiler room door. I think Reisen probably saw someone through it.

If you leave the door open, the sound can be heard all the way to the storeroom.

I ran out of the storeroom and over to the boiler room, and...

Right after I took the whiteboard inside, I heard Reisen's scream from the boiler room.

I found Reisen lying on the floor, covered in blood, with the murder weapon stuck in her neck.