Bruce Jenner -- 'Black Boxes' Could Hold Key to Fatal Crash

Bruce Jenner's fate could be determined in part by a metal box embedded in the twisted metal that was left on Pacific Coast Highway ... TMZ has learned.

We've learned the L.A. County Sheriff's Dept. is in possession of all 4 vehicles and the trailer that was attached to Bruce's Escalade. We're told Bruce's Escalade has a sophisticated "Black Box" that contains critical data ... including speed and braking info. The other 3 vehicles have similar data.

We're told the boxes gauge what the driver was doing for the 5 seconds before impact. In one-second intervals, the boxes show speed, braking, the position of the accelerator, etc.

We're told accident reconstruction experts can use the data from all 4 vehicles and determine the speed and distance between each of them. That could help determine with some precision if Jenner was following too closely behind the Lexus.

Many of the "Black Boxes" are activated by the airbag, but we're told even if the airbag does not deploy the information is still available.

As we reported, once the investigation is complete the Sheriff's Dept. will send the file over to the L.A. County D.A. for review -- which is SOP when someone dies in a car crash. If Bruce was following too closely, prosecutors could charge him with vehicular manslaughter, however we're told such prosecutions -- when the offense is following too closely -- are "very rare."