WILSON COUNTY, Tenn. — Federal investigators are evaluating evidence from the LifeFlight helicopter crash that killed one and injured two others.

The question remains: What cause the chopper to suddenly go down?

The safety requirements for medical evacuation helicopters like LifeFlight are among the most stringent in the industry.

Crashes happen, but they are rare.

So what happened ?

What we know is the Vanderbilt LifeFlight One helicpoter left it's base at Sumner County EMA at 1:33 p-m Saturday.

The weather was clear -- sunny skies and no wind.

Flight Aware Aviation tracking shows that not long after takeoff the helicopter reversed course to return to base and crashed eight minutes after it took off.

"Me and my husband were out of the deck and we seen this helicopter coming over t

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