Harvey Weinstein Lead Detective Under Investigation for Witness Tampering

The NYPD and the Manhattan District Attorney's Office have launched a new investigation in the Harvey Weinstein case ... and the target is the lead police investigator ... TMZ has learned.

Law enforcement sources tell us, the investigation centers around the case of Lucia Evans, who claims Weinstein forced her to engage in oral sex. The complaint against Weinstein alleges he grabbed the back of Evans' head and forced her mouth on his penis.

Our law enforcement sources say the witness -- a casting director -- says Evans told her she performed oral sex on Weinstein in order to get an acting role. Our sources say the lead investigator interviewed the witness who gave him a statement, but the investigator allegedly failed to disclose it to prosecutors.

Witness tampering generally requires some affirmative act to influence or pressure a witness ... but our law enforcement sources would not be more specific.

NYPD spokesperson Phil Walzak tells us, "The NYPD working with its partner DANY remains fully confident in the overall case it has pursued against Mr. Weinstein. The evidence shows that the criminal case against him is strong." The statement goes on ... "The NYPD will continue to assist the prosecution any way it can to ensure justice is achieved for these brave survivors."