A new study published in Energy Policy, an academic journal, suggests that hydraulic fracturing companies are becoming less willing to disclose the chemicals they inject into the ground.

Fracking has transformed the U.S. energy industry, but many people are concerned about the possible environmental impacts of the controversial drilling technique, such as increased seismic activity and water contamination.

To address public health concerns, 28 states require firms to report the chemicals they use in the fracking process. Of those, 23 states ask companies to disclose the information to the national registry FracFocus.

But Harvard University researchers Kate Konschnik and Archana Dayalu, who reviewed more than 96,000 disclosure forms on FracFocus, found that the amount of information withheld has increased the past three years.

The U.S. Environmental Protection Agency previously showed that between 2010 and 2012, 11 percent of the chemicals used in fracking were unreported. That figure rose to 16.5 percent between 2012 and April 2015, Konschnik and Dayalu’s study revealed.

The researchers said that their study is not an attack against fracking, but it should raise concerns.

“Fracking has helped put the United States on the trajectory towards energy independence,” said Dayalu. “But it doesn’t give the process a free pass against regulation.”

Konschnik called for policymakers to do more to ensure that citizens have access to complete and accurate chemical disclosures.

“We think states could signal to the oil and gas community that they take these disclosures seriously,” she said. “If companies don’t think regulators are taking this seriously, they won’t take the time and effort to make complete and correct disclosures.”