WASHINGTON (AP) - The owner of the Deepwater Horizon rig is accusing BP of withholding critical evidence needed to identify the cause of the worst maritime oil spill in history.

In a sternly worded letter obtained by The Associated Press, Transocean says that lack of information is preventing the company from informing state and federal investigators, and the families of those killed on the rig, about what happened.

According to the rig's owner, BP appears to be preventing any other entity from investigating the blowout.

Transocean also reveals that it reluctantly signed a confidentiality agreement at BP's request to obtain some limited data.

BP didn't immediately return calls for comment.