IIRC from [0] US politicians long regarded the horror stories out of Cambodia as exaggerated tales at best, enemy propaganda at worst, since the Cambodian regime was opposed to the newly formed Vietnamese government.
They couldn't politically support the cause of their former enemy, so Cambodians suffered for longer, until Vietnam got bold enough to invade and liberate them (Edit: I don't know whether the aim was to liberate, the aim probably was to invade but it resulted in liberation).
But yeah, I don't remember any material support either. But the political support was deciding in keeping the madmen going in the country.
[0] "A Problem from Hell" by Samantha Power