An investigation has concluded that a Navy SEAL who died in a parachute training accident in Florida last year became unconscious after exiting the plane.

The investigation also found that Petty Officer 1st Class William Blake Marston was unable to open his main chute, The Virginian-Pilot (http://bit.ly/1Qsy4Ag) reported. It also determined that Marston shouldn't have been on the plane to begin with because of a discrepancy over whether he was up to date with a required certification.

Rear Adm. Brian Losey wrote in his endorsement of the investigation's findings that investigators could not determine why Marston was unable to open the main chute.

"The most important question is unknowable, despite diligent investigative efforts," he said.

According to the investigation, Marston was supposed to open his parachute about five seconds after jumping off the plane, but that didn't happen. After 15 seconds he became unconscious.

The investigation also revealed the Marston did not have a current certification that is required to conduct a military free-fall parachute jump. His certification expired in March 2014. The lapsed certification should have kept him from jumping that day, the investigation determined.

Marston lived in Virginia Beach and was a member of SEAL Team 6. He died during the accident on Jan. 10, 2015, in Deland, Florida.

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Information from: The Virginian-Pilot, http://pilotonline.com