Hey, wait. Can't they know from the call's data whose phone it was made from?

I'm afraid not.

The phones used at the lab are a primitive model they developed themselves that can't identify the caller.

Also, there's an internal line in the night duty room.

Lecture room

Courtyard

Storeroom

Storeroom

Night duty room

Boiler room

Outside the lab

Window

Window

Window

Ringo

Reisen

Night duty room?

It's on the other side of the boiler room exit. It should've been empty at the time, though.

I see. So after attacking Reisen, the culprit left the boiler room and made the call from the night duty room.

They sound weirdly calm to have just killed someone, but I suppose they had locked the door behind them and were at no risk of being pursued.

So, any questions?

How much time passed between you leaving Reisen and hearing her scream?

Only about 15 seconds.

About 10 seconds from when I heard the boiler room door open, though. Judging from the time, Reisen might've been stabbed after exchanging only a word or two with the assailant.

5 sec

10 sec

*Ringo's internal clock