Tafsir of Al-Qamar 54:36

"And he had already warned them of Our assault..."

And he [Lot, peace be upon him] had warned them of Our assault—that is, Our severe taking [seizing] of them with punishment. It is also permissible that the intent behind it is the punishment itself.

"...but they doubted the warnings."

That is, they denied the warnings. They were in a state of doubt; thus, the verb is implicitly imbued with the meaning of denial, for without that, it would have been constructed with the preposition fi.