Safety inspectors are investigating whether seat belts on a model of Polaris Industries' Slingshot motorcycles are defective, allowing them to mistakenly release during crashes.

The National Highway Traffic Safety Administration (NHTSA) opened an investigation last week after a man was killed in Michigan last summer when the 2015 Slingshot he was driving hit another vehicle.

According to a police report, the three-wheeled cycle veered while making a lane change and slid into oncoming traffic where it was struck by a vehicle heading the opposite direction. The driver was wearing his seat belt and helmet when he was partly ejected from the Slingshot.

In a complaint filed with the NHTSA, the man's family claims the vehicle's seat belt retractor exploded during the crash and allowed the seat belt to release. The man reportedly died from a severed spinal cord.

"Because of the failure of this important safety equipment, our family was certain that Polaris would voluntarily recall all vehicles equipped with this obviously flawed safety device in an attempt to spare other families the heartbreak our family has suffered," the complaint said. "Obviously with no recall issued to date, we were incorrect in this assumption."

The NHTSA evaluation hopes to determine if a defect in the seat belts of possibly about 4,800 2015 Polaris Slingshots prevents them from properly restraining occupants during crashes.

Key Safety Systems Inc., a Michigan company, manufactured the seat belts for Polaris. Key Safety declined to comment on Monday. Investigators are unaware of any other seat belt retractor failures on Polaris Slingshots.

"We extend our deep condolences to the rider's family," Polaris said in a statement issued Monday. "At Polaris, the safety of our riders is our absolute top priority. We are fully cooperating with the NHTSA investigation, as well as conducting our own investigation, which is our standard practice."

The news of the investigation comes a little over a month since Polaris issued its 11th recall of the year. Its last recall affected about 2,800 of its Scrambler vehicles due to possible throttle-switch problems that could lead to crashes. In the last two-and-a-half years, Polaris has recalled more than 420,000 machines because of mechanical or fire problems.

Polaris' stock fell 69 cents a share to close at $106.69 per share Monday. The stock is up 30 percent since the beginning of the year.

Twitter: @nicolenorfleet