The two guys from the search party.

Just watched this movie again and loved it just as much as I did the first time I found it a few years ago. Something caught my attention this time around though...

There were two weird guys who claimed to be from the search party, looking for the killer of the girl in the park, who spoke to Barb and Phyllis. Just before the girls close the door on them, one of them makes a noise exactly like those weird grunting type noises heard during the obscene phone calls.

Could those two, or maybe one of them, have been Billy? It would explain why they were encouraging the girls to lock themselves in, if they had come from the attic they would easily have had the time to get back into the house and tuck themselves away up there before the girls locked everything up. Also, the police officer situated outside was killed, perhaps they were just caught out looking through the windows on their way back from killing him so they had to use the search party as a cover story to explain what they were doing lurking around outside.