10:55 AM PT — Law enforcement sources tell TMZ … Michelle Trachtenberg’s body may have rejected the liver transplant she received within the last year, causing complications. We’re told Michelle’s mother found her.

Michelle Trachtenberg, known for her roles in “Gossip Girl” and “Buffy the Vampire Slayer,” has died.

Police sources confirmed the shocking news to TMZ, though the exact cause of her death remains unclear. We’re told police and paramedics responded to a call for a woman in cardiac arrest, and found Michelle unresponsive. She was pronounced dead on the scene.

A separate source tells us the actress had received a liver transplant recently … meaning within the last year. At this point, police reportedly believe she died of natural causes.

We’re told Michelle was out with friends Thursday night of last week at Sartiano’s in NYC.

In recent months, she had posted photos where fans felt she appeared gaunt and frail.

In another post, she made it clear she never had plastic surgery, and was “happy and healthy.”

Michelle rose to fame as a child star in “Harriet the Spy” — later becoming a household name in her role as Dawn Summers on “Buffy the Vampire Slayer,” then took on the Upper East Side as Georgina Sparks in “Gossip Girl.”

A rep for Michelle tells TMZ … “It is with great sadness to confirm that Michelle Trachtenberg has passed away. The family requests privacy for their loss. There are no further details at this time.”

The NY Post was first to report her death.

Michelle was 39.

Michelle Trachtenberg’s untimely death at 39 has sent shockwaves through her fans and colleagues — and her former “Gossip Girl” costar Ed Westwick has posted a tribute to the star.

In a heartfelt post on his IG Story Wednesday, Ed shared a pic of Michelle as Georgina Sparks from the popular teen drama series … captioning it, “So sad to hear of the passing of @michelletrachtenberg.”

He added “sending prayers” with the prayer hand and a white heart emoji.

Governor Gavin Newsom‘s decision to ask the California Board of Parole to review the Erik and Lyle Menendez matter could make next month’s up-to-now critical resentencing hearing all but irrelevant.

As we reported, Newsom has asked the Parole Board to conduct a “risk assessment investigation” … this on the heels of TMZ’s “2 Angry Men” podcast in which Erik and Lyle appeared by phone to talk about what they’ve done with their lives behind bars for 35 years.

The Board will effectively conduct a parole hearing, in which they will assess whether the Menendez brothers pose an unreasonable risk to public safety. By doing so, the Board will necessarily review how Erik and Lyle have conducted themselves in prison.

Even if the Parole Board decides Erik and Lyle have been rehabilitated and pose no danger to society and recommends their release, it still has to be approved by the Governor. Fact is … Newsom has already asked the Board to conduct that review, so it will land on his desk regardless of what the judge does in the resentencing hearing.

The Governor already has a clemency petition on his desk, so it seems the decision on whether to free Erik and Lyle Menendez is now squarely in the hands of one man — Gavin Newsom.