(Reuters) - Injury rates have more than doubled at five West Virginia coal mines acquired by Murray Energy Corp. in 2013, according to a Reuters review of federal data, as the firm sharply increased the amount of coal produced per manhour.

FILE PHOTO: A coal barge moves past the at the Murray Energy Corporation port facility in Powhatan Point, Ohio, U.S., November 7, 2017. REUTERS/Joshua Roberts/File Photo

Although injuries and productivity rates rose over the same period, the causes of the increase in injuries remain unknown and could include a host of factors in the complex business of underground coal mining.

Murray - the nation’s largest underground coal mining company with about 6,000 employees producing more than 60 million tonnes of coal annually - bought the mines from rival CONSOL Energy. Those mines now account for more than half the firm’s production.

The injury rate at the five mines averaged 6.35 per 200,000 work hours last year, up from 2.79 in 2013, the last year CONSOL operated them, according to data from the U.S. Mine Safety and Health Administration. That’s the highest injury rate for the group in a decade and 70 percent above the national average for underground mines, the data show.

At the same time, coal output per manhour at the five mines rose nearly 50 percent since 2013 to 5.7 tons in 2017, according to MSHA figures. By comparison, the industry average in 2015 - the last year for which industry-wide data is available - was 3.4 tons per manhour.

Murray spokesman Cody Nett said that productivity has no correlation to injury rates. Strategies that Murray has used to increase output at the mines - such as continuing production through the overnight maintenance shift and keeping machinery and vehicles running during shift changes - are “standard practices widely utilized throughout the industry,” Nett said.

Nett said the injury rate at the former CONSOL mines was inflated by false claims from workers who took time off for other reasons. He also attributed the doubling of injury rates since Murray acquired the mines to under-reporting by the previous owner during the preceding years - an assertion disputed by the Department of Labor and its Mines Safety and Health Administration (MSHA), which collects the data.

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Regulators did cite CONSOL for under-reporting in a 2013 audit. But MSHA later revised the company’s data to include injuries that CONSOL initially failed to properly report, said MSHA spokeswoman Amy Louviere.

Asked about the agency’s adjustments, Nett said it was “flat out incorrect” that the mine safety agency had corrected CONSOL’s data.

CONSOL spokesman Zach Smith declined to comment on the company’s 2013 reporting violations.

The injury rate at the five mines translated to about 200 injuries requiring days off work in 2017. Workers at the five sites have suffered 159 injuries so far in 2018, according to the MSHA data.

Murray controls six other mines - in Kentucky, Ohio, Illinois, and Utah - and each has an injury rate below the national average, according to the data. The company has won numerous safety awards in recent years, including from the U.S. government’s National Institute of Occupational Safety and Health in 2015.

Davitt McAteer, a mine safety expert in West Virginia who had directed MSHA under former President Bill Clinton, said that many factors could have played into the increased injury rates at the Murray mines in West Virginia.

“Those particular mines are decades old, meaning miners are having to work deeper, more complicated coal seams, with aging equipment and infrastructure,” he said, adding that such conditions are more dangerous for workers.

The company’s CEO, Robert Murray, is among the coal sector’s most vocal advocates for rollbacks of environmental and safety oversight, frequently arguing that government regulation is unnecessarily onerous and costly for companies.

Murray Energy has also sued the government to invalidate an Obama-era rule called the Pattern of Violations rule, intended to beef up enforcement of safety standards against repeat offenders.

That case, filed in the U.S. District Court for the Southern District of Ohio, is now in settlement negotiations with the Trump administration.