EXCLUSIVE

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.