AUSTIN, Texas (AP) — A Texas sheriff said Thursday that three hog hunters from Florida died in an underground tank filled with sewer gas after one of them apparently tried rescuing their dog after it fell into the hole, followed by the other two jumping in to save them.

The bodies of two men and a woman, as well as the dog, were pulled from the tank in a cornfield on the rural outskirts of Austin. Bastrop County Sheriff Maurice Cook said the hole was a cistern with an opening roughly four feet (1.2 meters) wide and containing eight feet (2.4 meters) of water, as well as hydrogen sulfide gas.

He said the chain of events started early Wednesday in the middle of the night with one of the men apparently getting into the cistern to rescue the dog, which he described as a bloodhound. Clothing and boots belonging to the other two hunters were found near the hole, suggesting they removed them before also jumping in, Cook said.

He said authorities believe the hunters were overcome by hydrogen sulfide gas in the hole and sank to the bottom.

“There was no cover. This was just an open hole in the middle of a cornfield,” Cook said.

The victims were identified as Delvys Garcia, 37; Denise Martinez, 26; and Noel Vigil-Benitez, 45. All were from Florida.

Cook said the cistern had a “high level” of hydrogen sulfide. He said stagnating water and the decay of other animals that previously died in the cistern could create levels that would be deadly.

The hunting party included a fourth person, from Texas, who did not go into the hole. Cook said that hunter told authorities the dog escaped from their truck and that they tracked it using a device on the dog’s collar.

Efforts to recover the bodies were hampered by concerns from dive teams about the gas and the integrity of the structure’s walls, he said. The tank had “strong fumes, similar to those of a septic tank, coming from the cistern,” according to a statement from the sheriff’s office.