Not bombs. No planes have been observed, and the explosions aren't severe enough.

3 seconds sound clip from the Radar Men from the Moon - Moon Rocket (S1E1) movie soundboard.

You can hear this line at 00:03:08.318 in the Blu-ray version of the movie.

Quote context

[...]

- Since you people developed the rocket ship for us and we'll be the ones operating it, it has been decided that you should know more of our plans.

- I've been authorized to tell you this much: its primary use will be to combat this wave of sabotage and destruction of our defense system.

- You mean the government knows what's causing it?

- Not definitely.

- But tests of the wreckage show that the explosions are of atomic origin.

- You mean someone's dropping atomic bombs on us?

- Not bombs. No planes have been observed, and the explosions aren't severe enough.

- No, from what I've learned of the nature of the blast, they see more like an atomic ray of some kind. Of course that's just a guess.

- It's the same guess that we've made because it's the only possible answer.

- But no one has ever been able to make an atomic ray machine.

- You mean no one on earth ever has.

- Say, this rocket we're building we'll be able to fly to the moon, maybe someone up there's built one that could get down here.

- We believe that possible.

[...]