MLB Steroids Investigation -- Fingering Baseball Stars Forces Whistleblower Underground

The whistleblower who claims to have proof Alex Rodriguez, Ryan Braun and other baseball players took steroids ... has been threatened and hounded so much ... he's being forced into hiding, TMZ has learned.

According to sources connected to Major League Baseball's steroid investigation ... Porter Fischer has been under constant pressure for more than six months from private investigators -- hired by players, he believes -- attempting to get their hands on his records from the now infamous Biogenesis clinic in Miami.

We're also told Fischer's received death threats.

As we previously reported the ex-Biogenesis employee has documents which allegedly nail several players for purchasing and using performance enhancing drugs from the clinic within the last year.

He's currently negotiating a deal to turn over the docs to MLB officials.

Although Fischer stands to make a fortune -- possibly seven figures -- we're told he plans to use the money to disappear, and to protect his loved ones ... sort of like his own witness protection program.