He did not expect a similar fate, but the thought nagged at him.

During the journey back to Earth, a thought had repeatedly nagged him.

One thought nagged at him as he talked with them.

But one last thought nagged me and I gave it to him.

The longer he slept, though, the more the thought nagged at her.

He still wondered how much fuel remained, and the thought nagged at him.

A thought nagged at Ivan, and finally he recognized it.

The thought nagged at him: Why do such a thing, within the original home system of mankind?

He didn't want to believe it, not for a second, but the thought nagged him nonetheless.

Once again, the thought of going to them nagged at him.