NEW ORLEANS – Rescuers in helicopters and boats searched the Gulf of Mexico for 11 missing workers Wednesday after a thunderous explosion rocked a huge oil drilling platform and lighted up the night sky with a pillar of flame. Seventeen people were injured, four critically.

The blast about 10 p.m. Tuesday aboard the Deepwater Horizon rig 50 miles off the Louisiana coast could prove to be one of the nation's deadliest offshore drilling accidents in the last 50 years.

The Coast Guard held out hope that the missing workers escaped in one of the platform's covered lifeboats.

"We're hoping everyone's in a life raft," said Coast Guard Senior Chief Petty Officer Mike O'Berry.

Nearly 24 hours after the explosion, the roughly 400-by-250-foot rig continued to burn, and authorities could not say when the flames might die out. A column of boiling black smoke rose hundreds of feet over the Gulf of Mexico as fireboats shot streams of water at the blaze.

Adrian Rose, vice president of rig owner Transocean Ltd., said the explosion appeared to be a blowout, in which natural gas or oil forces its way up a well pipe and smashes the equipment. But precisely what went wrong was under investigation.

Crews were doing routine work before the explosion, and there were no signs of trouble, Rose said.

A total of 126 workers were aboard the rig when it blew up. The Coast Guard said 17 were taken by air or sea to hospitals. In addition to the four who were in critical condition, others suffered burns, broken legs and smoke inhalation.

Nearly 100 other workers made it aboard a supply boat and headed to the Louisiana shore.

The rig was tilting as much as 10 degrees after the blast, but earlier fears that it might topple apparently were unfounded. Coast Guard environmental teams were on standby, though officials said damage to the environment appeared to be minimal.

The rig, which was under contract to the oil giant BP, was doing exploratory drilling but was not in production, Transocean spokesman Greg Panagos said. Seventy-nine Transocean workers, six BP employees and 41 contract workers were aboard.

Ted Bourgoyne, a retired professor of petroleum engineering at Louisiana State University, said the explosion was probably caused by natural gas or a mixture of oil and gas coming through the well, combined with some kind of ignition source.

He said there are numerous defenses on a modern rig to prevent something like that from happening. For instance, fluids used in drilling are weighted with barium sulfite to prevent gas from traveling up the well, and there are alarms to alert workers to gas. Machinery is built to prevent sparking and is placed as far as possible from places where gas might leak.

"In almost all of these things, there's not one thing that happens; it's a series of things," Bourgoyne said.

Rose said the crew had drilled the well to its final depth, more than 18,000 feet, and was cementing the steel casing at the time of the blast.

"They did not have a lot of time to evacuate. This would have happened very rapidly," he said.

According to Transocean's website, the Deepwater Horizon is about twice the size of a football field. Built in 2001 in South Korea, it is designed to operate in water up to 8,000 feet deep, drill 5 ½ miles down and accommodate a crew of 130. It floats on pontoons and is moored to the sea floor by several large anchors.

"It's one of the more advanced rigs out there," Panagos said. He did not know how much the rig cost to build but said a similar one today would run $600 million to $700 million.

Workers typically spend two weeks on the rig at a time, followed by two weeks off. Offshore oil workers are typically well paid, earning $40,000 to $60,000 a year, more if they have special skills.

Last September, the Deepwater Horizon set a world deepwater record when it drilled down just over 35,000 feet at another BP site in the Gulf of Mexico, Panagos said.