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.

As we previously reported, Mike's friends and family also believe he died of a heart attack and do not blame the film for his death.

"The Lone Ranger" crew member who died on set last week worked alongside mega producer Jerry Bruckheimer and Johnny Depp for years, TMZ has learned.

Close friends tell TMZ the victim's name is Mike Bridger -- a welder and water safety guy who had 20 plus years in the biz ... and one of his biggest breaks was working with Bruckheimer on the set of the first "Pirates of the Caribbean" movie.

We're told Bruckheimer respected his work so much ... he hired him for every single "Pirates" sequel and then for "Lone Ranger." Our sources say he also became close with Depp while working on those films.

Close friends of Mike tell TMZ ... the family does not blame the production company for his death, and believes he died of a heart attack.

TMZ broke the story, Mike was working in a large tank on "The Lone Ranger" set when he passed away​. Law enforcement also believe his death was due to a heart attack. He was 48.

His funeral is currently being planned by family and friends.

1:10PM PST Law enforcement sources tell TMZ the movie wasn't being shot today, and the person who died in the tank was prepping it -- possibly cleaning it -- when someone noticed no bubbles from the gear were surfacing. That's when they pulled the man out of the water.

A Walt Disney spokesperson just confirmed our story, saying, "We regretfully confirm that a Lone Ranger crew member has passed away after being taken to a local hospital," adding "our full support is behind the investigation into the circumstances of this terrible event."

2:28PM PST Law enforcement sources tell us, it appears he had a heart attack. Investigators are treating it as a "natural death." An autopsy will be completed over the weekend.


A crew member working on the new Johnny Depp movie "The Lone Ranger" died this morning after drowning on the set and going into full cardiac arrest ... TMZ has learned.

Law enforcement sources tell us ... the man was a diver who was working in a tank on the Los Angeles set ... when something went wrong.

We're told someone on the crew called 911 to report a medical emergency ... and when responders arrived to the scene the man was in full cardiac arrest.

He was helicoptered to a nearby hospital, but EMTs were unable to restart his heart on the flight. Law enforcement sources tell us he was pronounced dead at the hospital.