Lisa Bloom -- who said she resigned from representing Harvey Weinstein after learning of his sexual misdeeds -- was deeply involved in investigating various female accusers to discredit them ... sources familiar with the situation tell TMZ.

We've learned Bloom actively investigated Rose McGowan back in July ... months before she left the case. We're told she had combed through private investigator's reports and discussed the findings with Harvey Weinstein and others.

We also learned Bloom investigated compromising information to use against another alleged victim, Ambra Battilana Gutierrez, specifically documents from Italy. We're told the endgame here was to kill a story. Gutierrez was mentioned in The New Yorker article.

We're told Bloom was also involved in lobbying the New York Times and NBC to back off their stories on Weinstein.

Our sources say Bloom had been extremely active in honing in on the alleged victims as early as April ... pouring over non-disclosure agreements and reviewing investigative reports prepared on at least one rape accuser.

Our sources say Bloom was also involved in efforts to secretly record at least one sexual assault victim.

We're told Bloom's involvement in the Weinstein sexual assault/harassment matters go back as far as January 2017.

When Bloom left the case she told the LA Times, "I was never aware that there were allegations of sexual assault. Should I have? Yes."