The culprit is the head of nurse Yuko Morita. She used the 4 WD car's muffler that her fiance is driving to send carbon-dixode gas into Egashira's room via the drain pipe, which she send sleeping gas in first. ฺำBefore committing the crime she went to Egashira's room to put a strand of hair to drain out all water from the sink's pipe. The evidence is that hamster was able to enter the room through draining pipe.
Morita confessed that she once caused a malpractice due to that she gave wrong medicine to people with same name and caused his death. Egashira accidentally know this so she silenced her. She just want to get married and be happy, which Conan complimented that "One cannot be happy over other suffering".
Afterward Conan and Ran discussed that Morita was really sympathetic as that malpractice day there were only three nurses and fifty patients. Kogoro then get a toothache again.
