The presence of the crater itself, its formation and characteristics, all support the theory that it was a meteor.
6 seconds sound clip from the It Came from Outer Space movie soundboard.
You can hear this line at 00:22:51.170 in the Blu-ray version of the movie.
Quote context
[...]
- Hello, Dr. Snell. Glad you could get away.
- I wouldn't have missed it for anything.
- Oh, you've just gotten started then.
- Well, hardly. I've seen enough.
- But I tell you, I saw a ship.
- You saw something that looked like a ship. You can't prove it, John.
- I can prove it if I can get you to help me dig it out of there.
- I can't in good faith ask the University to do it.
- We'd dig for months, and spend thousands, and maybe all we'd find would be these.
[...]