'The Lone Ranger' -- Government Org. Investigating Death On Set

An investigation has been launched into the shocking death of a crew member on the set of "The Lone Ranger" ... with a government agency vowing to leave no stone unturned until it discovers the truth.

TMZ broke the story ... a welder/water safety expert named Mike Bridger died on the L.A. movie set Friday while working inside a large water tank. Law enforcement sources tell us it appeared Bridger suffered a heart attack.

Now, we've learned Cal/Osha -- a workplace safety enforcement agency -- has launched an investigation to see if there was any negligence or wrongdoing on the part of Bridger's employer ... standard operating procedure following a death on a movie set.

In other words, Cal/Osha will look into how workers responded once they realized Bridger was unresponsive -- Did they follow proper emergency protocol? Was there a proper emergency protocol? etc.

A rep for Cal/Osha tells us the org. is also looking into the equipment and the tank ... and will also review the medical condition of the victim before his death. Cal/Osha will also conduct interviews with everyone who was on the set during the incident.

We're told there have been no official findings yet, and the final report could take up to 6 months to complete.