Rig operator PTTEP Australasia said no one was injured and nonessential workers were evacuated after the fire broke out on the West Atlas rig and Montara wellhead platform on Sunday.

The blaze started when workers began pumping heavy mud into a leaking well casing. An estimated 400 barrels of oil a day have escaped from the hole since Aug. 21.

Officials on Monday planned to pour more mud into the leak hoping to remove the source of fuel from the fire, which was sending massive plumes of smoke into the sky.

PTTEP Australasia chief financial officer Jose Martins said the company doesn’t know how the blaze started.

“Presently there are many unanswered questions, including what caused the fire,” Martins told reporters in Perth on Sunday. “Our sole focus now is the safety of all personnel, bringing the fire under control and completing the well kill.”

Federal Resources Minister Martin Ferguson said Monday that once the spill is contained he would launch an official inquiry.

“Our requirement is to assess the cause of the accident and any lessons to be learnt, and that could lead to a change in the regulatory environment,” he told Australian Broadcasting Corp. radio.

The oil slick from the rig, about 150 miles (250 kilometers) off Australia’s northwest coast, now stretches across thousands of miles (kilometers) of remote ocean. Indonesia said last week that thousands of dead fish and clumps of oil have been found drifting near its coastline.