The *degree* of asymptomatic transmission of novel coronavirus was (and, I think, remains) unclear. As to when we knew that it was happening, here's Dr Fauci on January 31.

Here's Fauci on that point March 3.

And here's Birx (Coronavirus task force coordinator) on March 14, addressing the how-much-not-whether question and how policymakers should proceed pending data.