The US Department of Justice has asked Transocean to preserve evidence in connection with the explosion and sinking of its Deepwater Horizon semi-submersible rig.

The accident has created one of the world’s largest environmental disasters, with more than 2.5 million gallons of crude spilling into the Gulf of Mexico, and led to the assumed deaths of 11 workers, still unaccounted for after the blast.

The Department of Homeland Security and the Interior Department are also investigating the cause of the disaster, the company said.

The Justice Department has reportedly sent letters to several entities that may hold important information regarding the incident.

Transocean’s Deepwater Horizon drilling vessel was under contract to BP at a the Macondo prospect, 40 miles from the Louisiana coast.

Meanwhile, Transocean has collected $401m from an insurance policy on the rig as partial payment, the company said.