Prosecutors have ruled that a New Jersey hospital executive killed his wife before setting their home on fire and killing himself.

The Somerset County Prosecutor's Office said Friday that a medical examiner ruled John Sheridan's Sept. 28 death a suicide. They say that the ruling supports their conclusion that he killed his wife Joyce before setting their home ablaze.

John Sheridan was the CEO of Cooper University Health System. He and his wife were found unresponsive during a fire in the master bedroom of their Skillman home.

The couples' four sons question the finding and say some key evidence is missing.

"This conclusion seeks to convict our father based on little more than rank speculation. We will be filing a lawsuit challenging the conclusion announced by investigators," said the sons in a statement. 

The prosecutor's office says some of the 180 interviews done during the investigation revealed John Sheridan had been withdrawn and acting unusually in the days leading up to their deaths.

AP wire services contributed to this report.