I am beginning to suspect that there is a sinister medical explanation for David's behaviour.

This is the sinister medical explanation for his behaviour.

On second thought, Sanders recognized that a far more sinister explanation for their departure from the hospital was at hand.

But there might be a more sinister explanation.

Now I began to think of another, more sinister explanation for his absence.

The complaint would seem to have a certain validity, although once again less sinister explanations are offered.

There could be an even more sinister, yet completely mundane explanation.

A more sinister explanation is that they were to stop the dead from rising.

Had those rotten flags simply chanced to give way beneath his weight, or was there a more sinister explanation?

There was another explanation, both sinister and plausible.