Durham, N.C. — State inspectors were investigating Wednesday how two workers for a Burlington company became trapped in a manhole in Durham Tuesday evening and died.

Jesus Martinez Benitez, 32, of Clayton, and Luis Castaneda Gomez, 34, of Durham, were working for Triangle Grading and Paving when they became trapped in a manhole at U.S. Highway 70, near Leesville Road, shortly before 6 p.m., authorities said.

Allen McNeely, deputy director of the state Division of Occupational Safety and Health, said that one of the men went down the manhole and suffocated when he was deprived of oxygen. The other man saw his co-worker was in trouble and went into the hole to rescue him and also suffocated, McNeely said.

"What you've got down in the bottom (of the hole) is any number of things – whatever is connected to that manhole, could be methane gas or could be any number of things – that would cause oxygen deprivation," McNeely said.

The Bethesda Volunteer Fire Department and the Durham County Hazmat Team tested the area before sending someone down into the manhole to extract the bodies and found that oxygen levels were too low to sustain life, said Capt. Don Ladd of the Durham County Sheriff's Office.

A passerby who called 911 to report the incident yelled down into the manhole at least twice but received no response from the men.

"I'm looking down in the hole, and these two guys are completely out," the caller told the 911 dispatcher.

The manhole led to an underground access point for utilities, according to Ladd, who described the space inside as a 4- by 6-foot concrete bunker about 10 to 12 feet below the surface.

Ladd said it was unclear what the men were doing in the manhole, although they might have been trying to remove a piece of equipment or open a valve.

Durham is adding water lines along U.S. 70 in the area, officials said.

Eight workplace health and safety complaints have been filed against Triangle Grading and Paving in the past 14 years, according to U.S. Occupational Safety and Health Administration records. One worker was killed in 1997 when a hydraulic hose ruptured and caused the bulldozer he was operating to catch fire, records show.

The company was fined $40,000 last year for serious and willful safety violations for exposing employees to hazards while working in an 8-foot-deep trench at Fort Bragg, records show.

"An employer is responsible for making sure confined spaces are monitored, so you know what's going on down there," McNeely said. "That's some of the things we'll be checking on, that and training the individuals, making sure they knew what they were working on."

Triangle Grading and Paving officials issued a statement Wednesday expressing sorrow at the men's deaths and stating that the firm is dedicated to ensuring worker safety and is cooperating in the state investigation.

"We take great pride in our safety and education programs at Triangle. We do everything in our power to prevent injuries and deaths in a dangerous occupation," the statement said.