MORGANTOWN, W.Va. (AP) — A spark from a natural gas drilling operation in north-central West Virginia ignited methane gas several hundred feet underground early Friday, sending up a fireball and triggering a blaze that officials said burned for about an hour on the floor of the rig.

Three workers were injured, two seriously enough to be airlifted to a hospital after the fire at the Antero Resources site near Sycamore in Harrison County. The fire was quickly extinguished and the well pad was in a rural area, so it posed no danger to the public.

Two victims were flown to Ruby Memorial Hospital in Morgantown, said Sgt. Heather Mick of the Harrison County 911 Center. A third was transported by ambulance.

Their conditions weren't immediately available, but state Department of Environmental Protection spokesman Tom Aluise said one had returned to the job site by 9 a.m.

Workers were in the early stages of drilling a Marcellus shale gas well, Aluise said. The drill was about 400 feet deep when they began to withdraw it, creating a spark that ignited the methane. That created more of a fireball than an explosion, he said.

The accident happened at the Cottrill No. 3 well on Antero's Southern pad, and Aluise said the crew doing the work was with Hall Drilling LLC of Ellenboro.

Neither Hall Drilling nor Colorado-based Antero immediately returned messages Friday.

Aluise said Antero voluntarily shut down the operation, and a DEP investigation is under way.

The rig was damaged badly enough that a new one may need to be brought in "if and when they resume drilling," Aluise said.

In June, another Antero drilling operation triggered several backyard geysers when workers struck an aquifer in the Sardis area and inadvertently re-pressurized a handful of old water wells. Emergency management officials and residents said some were 10- to 12-feet high.

There was no interior damage in the affected homes. The residents' wells had long been disconnected from indoor plumbing because their homes are all on a public water supply.

On July 31, the DEP ordered Antero to provide a detailed incident report, including a chart outlining the pressures involved, a list of the water wells that were affected and the current status of those wells.

The DEP also wants pre- and post-water analyses for each of those wells, along with a map showing their locations in relation to the well pad.

The letter from Office of Oil and Gas Director James Martin also demands information about any water wells that Antero drilled and a report that cites "any direct or indirect cause" and lays out what Antero will do in the future to minimize the likelihood of another incident.

Antero vice president Al Schopp said at the time that workers were drilling an initial hole with just fresh water and air when the bit became stuck. Rather than turn the air flow off, the crew left it on as they tried to withdraw the drill. That recharged the aquifer and trapped the air, which then sought a place to escape.

That accident happened thousands of feet above the targeted oil and gas deposits.