An unimaginable tragedy struck in a small community of a few dozen homes on Long Key Road in the Florida Keys Monday morning, when three workers from a private contractor tasked with fixing a roadway climbed into a hole in the ground and, ultimately, to their deaths. By the time a Key Largo firefighter climbed into the same hole near Lake Surprise in a desperate attempt to save the men, they were dead. And within seconds, the firefighter was also overcome by poisonous gas and was fighting for his life. The hole, just wide enough to fit a body and about 15 feet deep, was filled with hydrogen sulfide and methane gas created from years of rotted vegetation. It was so poisonous, Monroe County Sheriff Rick Ramsay said, that the firefighter was unconscious within seconds. "Neighbors were complaining about a sewage backup," Ramsay said. "So they went to investigate." Ramsay's description of the tragedy was like a stack of dominoes cascading downward. The first man, the sheriff said, removed a manhole cover and went underground. Then silence. A second man climbed down in search of his coworker and he, too, lost consciousness. Desperate, a third man climbed into the same hole and was immediately overcome by fumes. None of the four men wore masks or carried the vitally important air packs that likely would have saved their lives. Ramsay said his detectives and workers from the Occupational Safety and Health Administration will investigate the deadly incident. The names of the three dead men - who work for a private Michigan-based roadwork contractor named Douglas N. Higgins - were not immediately released. Neither was the identity of the Keys firefighter who was taken to Mariners Hospital and then flown to Jackson South Medical Center. "We pulled the unresponsive firefighter out with the help of two sheriff's deputies. He was nonresponsive. Not breathing. They used CPR and revived him," Ramsay said. "I believe he's in a coma." Firefighters were able to remove two of the men's bodies. A third was removed later in the day with the help of Miami-Dade Fire Rescue. Also taken to local hospitals were three Monroe County sheriff's deputies who complained of dizziness. The neighborhood was ordered evacuated until determined safe.