I’m looking for a movie I watched as a kid in the ’60s.  It’s plot was similar to “The Snorkel”, where killer would hide under the floor with an air bottle and kill the room’s occupant with gas.  I want to say the action took place at a seaside resort.  Movie ends with little girl who figures out how the killer is doing the murders asking her parents to search room where latest murder occurs.  She figures out where murderer is hiding under the floor, and asks father to look behind a wardrobe, moving the wardrobe on top of the killer’s hiding place.  Knowing she’s now trapped the murderer, she then leaves with her parents, leaving the murderer trapped, presumable to die trapped under the floor.