Story via TMZ - Diddy is the subject of a secret, NYPD investigation, and although it's unclear if his ex, Cassie Ventura, made the criminal complaint, there is strong evidence it involves sexual assault.

Law enforcement sources tell TMZ ... there is an open case at NYPD with the name, "Sean Combs." We're told it is an active case but the case file has been "locked" -- essentially meaning it's a sensitive case where access is restricted.

As we reported, Cassie has filed an explosive federal lawsuit, which alleges rap* and human trafficking, among