Denver Broncos defensive end Antonio Smith is under investigation over a criminal allegation that is "sexual in nature".
It had previously been reported that an investigation had been launched centring around child abuse allegations.
Smith has not yet been arrested and no charges have been filed, although the investigation is understood to be ongoing.
"Last night, we became aware for the first time of an allegation involving Antonio Smith," the Broncos said in a statement.
"We are now in the process of gathering more information."
The 33-year-old signed a one-year deal with the Broncos in April, after starting 16 games last year for the Oakland Raiders.
