The cause of the accident is unknown.
The accident is assignable to human error.
The accident originated from carelessness.
Accidents are inevitable.
The accident happened, but they couldn't help it.
The cause of the accident is still obscure.
It is a pity that nobody was saved in the accident, isn't it?
No one knew for certain how the accident happened.
The cause of the accident is a complete mystery.
It is I that am responsible for the accident.
No one knew for certain how the accident happened.
The effects of the accident were not serious.
I told nobody about the accident.
I don't blame you for the accident; it was not your fault.
I don't blame you for the accident. It wasn't your fault.