A BP drilling engineer who was a key decision maker at the rig in the Gulf of Mexico that blew up in April has refused to testify before a federal panel investigating the incident.

Mark Hafle exercised his constitutional right not to testify on Friday.

The U.S. Coast Guard and the Bureau of Ocean Energy Management, Regulations and Enforcement have been holding hearings since Monday in Houston. Their goal is to determine what caused the explosion on the Deepwater Horizon rig that led to the largest offshore oil spill in U.S. history. The panel also will make recommendations to prevent such a catastrophe in the future.

BP drilling engineer Brett Cocales is testifying before the panel on Friday.