PORTSMOUTH, N.H. — A New Hampshire brewery has hired an independent investigator to help determine what caused a keg explosion that killed an employee of the Redhook Ale Brewery in Portsmouth.

Redhook’s parent company, Craft Brew Alliance, Inc., says officials haven’t determined what caused the plastic keg to explode while an employee was cleaning it with compressed air.

The April 24 explosion killed 26-year-old Ben Harris of Newington.

Foster’s Daily Democrat reports a preliminary investigation concluded Harris was in the process of emptying the keg in order to stack it on a pallet when it blew apart.

The alliance says the keg involved in the accident was not owned by Redhook, but had been delivered to the brewery by mistake.