HOUSTON (Reuters) – The search continued for a worker missing from an oil production platform in Louisiana’s Lake Pontchartrain that exploded and caught fire late Sunday, sending six others to hospital with injuries, the U.S. Coast Guard said.

Coast Guard rescue crews were battling 4 foot to 5 foot (1.2-1.5 meter) waves on the lake and a helicopter joined the search at daybreak after an overnight effort failed to locate the worker, officials said at a briefing on Monday.

Names of the missing and injured were not disclosed.

Three workers remain in hospital for blast and burn treatment, some in critical condition, Mike Guillot, emergency medical services director at East Jefferson General Hospital, said during a news briefing on Monday. Four others were released overnight.

Firefighters managed to turn off a valve that was feeding natural gas to the platform and feeding the blaze, Dave Tibbetts, chief of the East Bank Consolidated Fire Department, said at the same briefing. He said the fire would eventually burn itself out as the gas stopped flowing.

“We feel confident from the fire fighters’ aspect that we have control of the situation at the present time,” Tibbetts said.

The platform is operated by Clovelly Oil Co, Donald Mackenroth, a Clovelly vice president reached by phone, said. Details of what happened on the platform are still to be determined, he said on Monday.

There were at least eight people on the platform and on a nearby supply vessel at the time, officials said. An investigation into the explosion and fire is starting, they said.

There was no sign of an oil sheen or other environmental damage on Monday, but officials said they were still evaluating the site. The platform is used to transfer and store oil and natural gas from nearby wells, the officials said.

Police were notified of a loud explosion about 7:18 p.m. local time by residents, and soon after received a call from the platform that a boat carrying injured was on its way to shore, said Kenner, Louisiana, police Lieutenant Brian McGregor.

East Jefferson General Hospital, which received some of the injured, declined to comment on the number of treated or on the type or extent of their injuries.

Clovelly is the only oil company operating in Lake Pontchartrain, according to its former president, Harald Werner. It has two or three active oil and gas wells at the site, he said.