Since 2006, at least 74 workers have died in the Bakken oil fields, an area that stretches across Montana, North Dakota and the Canadian provinces of Manitoba and Saskatchewan, according to data obtained from Canadian and U.S. regulators.

Montana and North Dakota account for 60 of those fatalities. But that number is likely an undercount, as data from the Occupational Safety and Health Administration does not systematically account for all fatalities related to the oil and gas industry. It also does not cover any independent contractors.

The initial oil and gas industry data that OSHA provided to Reveal for our investigation into worker safety accounted for 47 fatalities in Montana and North Dakota. Those fatalities were tagged with a handful of North American Industry Classification System codes directly related to oil and gas.

But we suspected more workers died doing jobs that helped propel the boom because many workers have been drawn the Bakken to perform jobs to help expand the area’s infrastructure. We reviewed the data with OSHA multiple times to account for people who had died doing jobs in support of the industry, such as companies excavating roads or constructing well pads for oil and gas operations.

Each round of discussion turned up new records that previously were missed. One example: An employee of A&R Construction, a pipeline contractor based in North Dakota, died when he fell off a trailer carrying pipes, including some for oil and gas operations, according to OSHA records. OSHA officials initially concluded that the worker’s death wasn’t directly related to the oil and gas industry because the company was devoted primarily to construction and delivery goods “without regard to the product being delivered,” said Eric Brooks, director of OSHA’s Bismarck office, which covers North Dakota and South Dakota. We included the worker’s death in our count because Brooks said some of the pipes on the trailer were intended for an oil and gas project. After several interviews with OSHA and more analysis, we included 13 additional fatalities with our original count.

Those fatalities might have been overlooked because the North American Industry Classification System emphasizes the nature of the business over the specifics and nuances of the job being done.

“If you’re just relying on the NAICS codes, as OSHA does agencywide, you’re going to miss a whole bunch of cases that are related to oil and gas,” Brooks said.

Still, Reveal’s final tally may not reflect the full extent of the dangers in the Bakken. OSHA’s authority prevents it from investigating incidents involving independent contractors or workers who die in most vehicle accidents. Our total includes the deaths of at least two independent contractors who we learned through our research had died over the last few years.