Menendez Brothers Watched Murder Case Update From Prison

Erik and Lyle Menendez were glued to the screen as prosecutors made a major announcement in their murder case ... watching it all unfold from behind bars.

Sources with knowledge of the situation tell TMZ ... the famous convicts watched Thursday's press conference live from prison, tuning in to see L.A. County D.A. George Gascón announce prosecutors would be reviewing new evidence.

Gascón says his office is reviewing Erik and Lyle's claims that their father, Jose Menendez, molested them before they killed him ... and he said he would decide if the brothers should get a new trial or if prosecutors should go for a new sentence -- presumably manslaughter -- which could set them free.

Our sources say Erik and Lyle have been confident -- even smug -- that their case would get a second look ... and they're just as confident they will get out of prison.

The brothers are serving life sentences at Richard J. Donovan Correctional Facility near San Diego ... and they've got lots of folks calling for their release, including Kim Kardashian.

Gascón said Thursday his office is aware of the new claims by a former member of the band Menudo -- who says Jose also molested him.

Those claims are highly relevant, because it goes to whether the judge in the second Menendez brothers trial erred by not allowing evidence in that Jose allegedly molested his sons.