Old movie with someone with scuba tank hiding under floor

Hello! I remember seeing an obscure black & white film many years ago. I don't know the title, but would like to find it so I can see it again.

I don't think that it had any big stars in it, and I'm guessing it was made in the late 1950s or early 1960s.

One memorable scene was where someone with a scuba tank was hiding in a room where someone was sleeping. The person with the air tank taped the windows and doors shut and turned on the gas, then hid under a trap door in the floor. Later, the person who was sleeping was found dead of asphyxiation and it was assumed to be a suicide (of course, the person with the scuba tank stayed safe and later emerged from the room after the body had been removed.

I know it's kind of obscure, but hope that someone might recall this. Thanks!