A drilling rig continued burning out of control in the Gulf of Mexico on Wednesday, raising questions about safety improvements that were supposed to follow the 2010 Deepwater Horizon disaster.

The rig, owned by Houston-based Hercules Offshore , has been unmanned since Tuesday morning, when a natural-gas well that it was drilling blew out, enveloping the rig in a cloud of gas that ignited late Tuesday night.

The crew tried to shut down the well using safety equipment known as a blowout preventer, which are valves meant to seal off pipes leading from oil and gas reservoirs, according to Hercules. But workers didn't succeed in doing so before they had to evacuate for their own safety, the company said.

The cause of the blowout, which occurred about 55 miles off the Louisiana coast in 154 feet of water, is still under investigation, according to the companies involved and federal regulators. They are also trying to figure out what ignited the gas from the well owned by Walter Oil & Gas Corp. of Houston.

Probes into the Deepwater Horizon explosion three years ago found that the blowout preventer aboard that drilling rig failed to work properly, allowing millions of barrels of oil to leak into the Gulf in the worst offshore oil spill in U.S. history. Investigators called for improvements not only to blowout-prevention equipment, but also to overall safety practices offshore.