Complex sentence: The cause of death was later speculated in the official police report to have be a cervical spinal cord injury that caused cardiac arrest; however, Misawa's family invoked a Japanese law that requested the police not publicly release the official cause of death.
Split into simpler sentences: The cause of death was later speculated on in the official police report to have been a likely cervical spinal cord injury which caused cardiac arrest. However, the official cause of death will probably never be known since Misawa's family invoked a Japanese law which allowed them to request that the police not publicly release the cause of death.