A new report charges that several oil and gas companies have been illegally using diesel fuel in their hydraulic fracturing operations, and then doctoring records to hide violations of the federal Safe Drinking Water Act.

The report, published this week by the Environmental Integrity Project, found that between 2010 and July 2014 at least 351 wells were fracked by 33 different companies using diesel fuels without a permit. The Integrity Project, an environmental organization based in Washington, D.C., said it used the industry-backed database, FracFocus, to identify violations and to determine the records had been retroactively amended by the companies to erase the evidence.

The Safe Drinking Water Act requires drilling companies to obtain permits when they intend to use diesel fuel in their fracking operations. As well, the companies are obligated to notify nearby landowners of their activity, report the chemical and physical characteristics of the fluids used, conduct water quality tests before and after drilling, and test the integrity of well structures to ensure they can withstand high injection pressures. Diesel fuel contains a high concentration of carcinogenic chemicals including benzene, toluene, ethylbenzene, and xylene, and they disperse easily in groundwater.