Actually the US was well aware, because China confirmed to the world on 20 january that the virus spread person to person. The White House Proclamation on 31 January suspending travel from China actually refers specifically to that fact. It also refers to the fact that person to person spread had already occurred within the US.
The fact the US knew the virus spread in the community, and that it was already in the US, means they knew the travel restriction alone could never achieve much by itself. Yet rather than start to prepare the population for the prospect of a significant impact, and using the time to start manufacturing equipment etc, the US doesn't seem to have done much until March, by which time plainly things had got out of hand. Wasting that precious time is really the main criticism I've heard about how the US has handled it all.