Tsumetai Misshitsu to Hakase-tachi

On a colleague, Professor Kita's, suggestion, Associate Professor Saikawa Souhei and his student Nishinosono Moe head to Kita's research institute, where various experiments are being conducted in a low-temperature research lab.
During the night a murder takes place: two graduate students are found dead in the lab.
How did the killer enter the locked room? How did they leave?