Searchers headed out for a third day Thursday morning to look for a 30-year-old hunter who walked away from his camp Monday in Big Cypress National Preserve in Florida.

Preserve spokesman Bob DeGross says the search for Jamey Mosch is focused in a swampy section of the Bear Island Unit of the preserve in an area north of Interstate 75 and east of Turner River Road. On Wednesday traffic on the interstate was briefly halted to eliminate background noise after searchers thought they heard a weak call or a whistle, Mr. DeGross said Thursday morning.

On Wednesday, crews came upon a footprint thought to be from the missing hunter, as well as a "ThermaCell," which is a mosquito repellent device that works by dispensing repellent into the air. On Thursday the plan was to have three or four helicopters and upwards of 50 ground searchers looking for Mr. Mosch, the park spokesman said.

Mr. Mosch, who recently moved to Florida from New York, disappeared Monday when he walked away from a hunting camp he was sharing with five or six others. Complicating the search is that he was dressed in green, officials said. The area also is habitat for alligators and water moccasins, a poisonous snake.