Two skydivers have been killed after their parachute apparently failed to open, according to US authorities.

The victims, who have not been identified, were found in a vineyard in Acampo, California, after the local sheriff's office received a call on Saturday morning.

Witnesses in nearby Sacramento said they did not see a parachute open.

San Joaquin County Sheriff's Office Lieutenant, Randy Johnson, said the relationship between the victims was unclear, but amateur skydivers are often required to jump with an instructor.

The skydivers were jumping with the Parachute Center skydiving school in Acampo.

It's owner, Bill Dause, told NBC News the instructor involved was an independent contractor but highly experienced.

He said: "The parachute failed to eject properly. We have no explanation why. The only thing it looks like is something may have gone out of sequence. That may have caused the problem".

The Federal Aviation Administration is investigating the incident.