KEY LARGO, Fla. – Three workers in the Florida Keys died Monday morning after they were overcome by fumes, authorities said.

Sky 10 was above Long Key Road, near mile-marker 106 in Key Largo just after 10:30 a.m., where tarps were visibly covering two of the bodies.

Miami-Dade Fire Rescue officials said they responded to reports of three people down.

The victims were working at a road project for the project's general contractor, Douglas N. Higgins, county spokeswoman Cammy Clark said.

Clark said a worker went inside a drainage manhole to see why the newly-paved Long Key Road was settling at that location.

She said the worker got trapped inside the manhole and thee other workers, a volunteer firefighter with Key Largo Volunteer Fire Department and two Monroe County Sheriff's Office deputies tried to help get him out.

Two workers who collapsed, and the firefighter, who also collapsed after going underground, were pulled from the hole, authorities said.

The two workers were pronounced dead at the scene. It took authorities several hours to recover the body of the third worker.

Clark said the firefighter and deputies were taken to Mariners Hospital in Tavernier.

The firefighter, identified by relatives as Leonardo Moreno, was then airlifted to Jackson Memorial Hospital's Ryder Trauma Center, where he is listed in critical condition.

A GoFundMe page has been created to help with his medical expenses.

"A firefighter had an air pack on," Monroe County Sheriff Rick Ramsey said. "He found the hole too small, so he elected to take his air pack off and go inside the hole to attempt the rescue."

The deputies are being treated for non-life-threatening ailments.

A fourth worker for the contractor was treated at the scene.

Clark said the cause of deaths will be determined by the Monroe County medical examiner.

A woman who lives near the manhole told Local 10 News that the area has smelled of rotten eggs for the past couple of months.

"It smells like rotten eggs," Barbara Guerra said. "It was out here again this morning and I'm used to it because they've been a whole year already, but yeah, the smell was very strong this morning."

The contracted workers were in a 15-foot hole and it's believed that a build up of hydrogen sulfide and methane is to blame for the deaths.

"There's no sign of any pre-venting going in, and obviously going into a contained environment where there is gases can be deadly, as we unfortunately found out today," Ramsey said.

Monroe County Sheriff's Office spokeswoman Becky Herrin said five homes in the area have been evacuated.

Records show that the contractor was fined for an incident at a manhole in Collier County in 2002.

In that case, OSHA said workers were exposed to hazardous conditions. An employee who answered the phone at Douglas N. Higgins declined to comment on the incident.

The workers killed Monday were identified as Elway Gray, 34, of Fort Lauderdale, Louis O’Keefe, 49, of Little Torch Key, and Robert Wilson, 24, of Summerland Key.

"His daughter was his entire universe. He would give anything for her or her mother," Elway Gray's sister, Cerene Gray, said.