Governor Gavin Newsom is ordering an investigation into whether Erik and Lyle Menendez are fit to return to public life … TMZ has learned, and he says it’s a critical first step before he, or anyone else, can decide if they get out of prison.

This is a major, unexpected development in the Menendez brothers‘ fight for freedom. It seemed the Governor was going to take a back seat until the courts ruled on Erik and Lyle’s fate, but all that has now changed.

The CA Gov. tells us, he’s directing the Board of Parole Hearings to conduct a “risk assessment investigation” on the Menendez bros., which he says is their version of a “parole hearing” — but before the Board weighs in, a “Comprehensive Risk Assessment” must be done.

Newsom says experts will analyze “what led each brother to commit their offense, and if they have done the transformational work needed — and gained the skills — to avoid repeating their mistakes.”

The Governor’s initiative comes one week after Lyle and Erik appeared on TMZ’s “2 Angry Men” podcast, speaking with Harvey and Mark Geragos (their attorney) about their lives during incarceration … including several programs they’ve launched to better the lives of their fellow inmates.

It sounds like all that information will be factored into Newsom’s decision — along with the Risk Assessment, which will help him determine just how rehabilitated the Menendez brothers are after 34 years in prison … and, ultimately, whether he should commute their sentences.

That’s one path to release, but the Gov. says the investigation results will also be shared with the Board of Parole and the judge deciding whether they get resentenced … which is their other potential path to freedom.

Once the Risk Assessment is complete — which will take a few months — Newsom says there will be a hearing “that works like a standard parole hearing, providing the District Attorney and the victims a chance to be a part of the process.”

L.A. County D.A. Nathan Hochman made it clear last week he’s opposed to the Menendez brothers getting a new trial, but told us on “TMZ Live” he’s open to the possibility of them getting resentenced.