'cause unknown' is correct and usable in written English.
You can use it when the cause of something is not known or understood. Example: The cause of the accident is 'cause unknown'.
I have unstable BP with the cause unknown.
The diagnosis was acute liver failure, cause unknown.
The epidemic of cholera, cause unknown and prognosis dire, had reached its peak.
Type 1 is an auto-immune disease, cause unknown (although genetics is believed to play a part).
Many cases were marked suicide or cause unknown, but past police conclusions are now being called into question.
His mother later died, of causes unknown.
This is because the fading channel causes unknown phase shifts in the received signal.
In the beginning of my teen years, my hearing loss changed drastically, causes unknown.
Since 1992, the county coroner has investigated seven Does, the victims of car accidents, violent crimes, suicides or causes unknown.
The majority (>85%) of PD cases are sporadic, and the underlying molecular causes unknown.
