A major brush fire on Big Pine Key continues to smolder and burn for a fourth day. Authorities say they don't know how it started and a state agency has taken over the response.

As of midday Wednesday, 50 percent of the fire has been contained and one fire official was optimistic that the worst is behind Big Pine.

“It’s pretty much contained,” said Jim Callahan, Monroe County Fire Rescue chief. “We’ve got plenty of resources here now. County fire is stepping back.”

Containment means that half of the fire's perimeter has been worked on to prevent it from growing.

Meanwhile, the Florida Forest Service has taken the lead from Monroe County Emergency Services. The state agency says 96 acres have burned and 46 firefighters are on the job on an island about 30 miles north of Key West still recovering from devastation left by Hurricane Irma last September.

“There is still an active fire,” said Melanie Banton, a spokeswoman for the fire response who works for the Forest Service. “It may not be an open flame but there is still heat in that area."

Banton said crews will continue to work on the fire at least through the week.

The state has brought in two "bombardiers," which resemble bulldozers on tracks that carry 700 gallons of water. Also, five “type 6” engines, which are like Ford 350 pickups holding big water tanks, are on the scene.

“Things are starting to look better,” Banton said Wednesday. “However, there is also going to be a shift in the weather this afternoon. We’re expecting the winds to change. It could push embers in a different area.”

Callahan said the fire's cause remains under investigation, but officials have ruled out a couple of ways.

"At this point we know it wasn't started by lightning or any natural way," Callahan said. "We haven't pinned down who started it or how it started."

"Today with the addition of more equipment and crews, resources will continue to patrol and monitor the fire's edge and improve containment lines," the Forest Service said in an update Wednesday. "Crews will also continue mop-up operations to put out hot spots and smoldering stumps and trees."

Keys residents can expect additional columns of smoke as temperatures rise and the humidity decreases. People who haven't seen smoke before may see it today, Callahan said.

The county needed state and federal agencies to step in to help handle the brush fire. Some firefighters from the Georgia Forestry Commission have also come down to help.

“We didn’t have the resources or equipment,” Callahan said. "We don’t have off-road firefighting equipment. We don’t have bulldozers or brush trucks.”

The helicopters seen dropping 800-gallon loads of water on the fire come from the state.

There are no closures or evacuations ordered.

The National Weather Service predicts no rain in or around Big Pine through Thursday.

“Water isn’t always our best way of putting out fire,” Banton said. “We use rakes, bulldozers, axes. We do use water to cool things down but even the water might not completely put out a smoldering stump.”

Many residents of Big Pine remain displaced by Irma. Now, other locals have had to leave their homes because of the brush fire.

Callahan said no one is displaced, except for the man who lost his home to the fire.

"All electricity is back on," he said. "We’re just asking people to pay attention to their surroundings."

Said Banton: “We’re not going to leave without making sure we’ve exhausted every effort to make sure this fire is 100 percent contained."