Greg Biffle‘s estate was hit with two multi-million dollar wrongful death lawsuits … stemming from the tragic airplane crash that claimed the life of the NASCAR great and six others.

The estates of Dennis Dutton — who was piloting the Cessna Citation II — and his son Jack filed the suits earlier this week in Iredell County, North Carolina, claiming Biffle was ultimately responsible for the fatal accident on December 18.

The lawsuits allege the crash was caused, at least in part, by “inadequate maintenance and upkeep,” which the Dutton estates claim fell to Biffle as the aircraft’s owner.

Of course, the light corporate jet crashed just minutes after taking off from the Statesville Regional Airport … erupting into a ball of flames as it impacted about a quarter of a mile before the runway they’d just taken off from 10 minutes earlier.

The plane experienced issues almost immediately, with cockpit voice recorder data indicating a thrust issue during the takeoff roll.

Sadly, the decision was made to press forward with the flight.

Preliminary information suggests that the lack of a qualified co-pilot may have contributed to the tragic outcome, as no one other than Dennis Dutton was rated on the Citation II, despite the fact that Jack and Greg were licensed pilots.

The National Transportation Safety Board has not yet released its final findings.