The requirements changed on the 1st of May. Below is what is on the UK Government website. I'm pretty sure it will apply to US citizens but please double check.
From 1 May travellers will no longer be required to complete a passenger locator form (EU PLF) to enter or travel through Italy as a visitor.
Until 30 May, all travellers, aged 6 and over, must show one of the following:
proof of vaccination OR
evidence of a negative PCR test taken within 72 hours before entering Italy or a negative rapid lateral flow test taken within the 48 hours before entering Italy OR
a COVID-19 recovery certificate, certifying that you have recovered from COVID-19 in the last 6 months