The Menendez brothers followed updates on the murder case from prison

Eric And Lyle Menendez sat transfixed in front of the screen as prosecutors made a major announcement in their murder case…and watched it all from behind bars.

Sources with knowledge of the situation tell TMZ … the famous convicts watched Thursday's press conference live from prison and tuned in to see the LA District Attorney George Gascon announced that prosecutors would examine new evidence.

Gascón says his office is examination Erik and Lyle's claims that their father Jose Menendezmolested them before they killed him … and he said he would decide whether the brothers should get a new trial or whether prosecutors should impose a new sentence – presumably manslaughter – that could set them free.

According to our sources, Erik and Lyle were confident – even smug – that their case would be reconsidered… and they're equally confident that they'll get out of prison.

The brothers are serving life sentences at the Richard J. Donovan Correctional Facility near San Diego… and many people are calling for their release, including Kim Kardashian.

Gascón said Thursday that his office is aware of new allegations from a former member of the band Menudo who says Jose also harassed him.

These claims are extremely relevant as they determine whether the judge in the Menendez brothers' second trial erred by not allowing evidence that Jose allegedly abused his sons.