Texas oil and gas regulators say there is no evidence that a record 4.0 earthquake last month in the northern part of the state was caused by injecting oilfield waste into underground wells.

The Texas Railroad Commission said Friday it reached that conclusion after testing five disposal wells in Johnson County. The May earthquake didn't cause any serious damage or injuries. It was the largest in recorded history in North Texas.

The findings come as the powerful state agency is asking natural gas companies to begin proving their operations aren't to blame for earthquakes. More than 50 have rattled North Texas since late 2013.

Injection wells store wastewater from hydraulic fracturing. That has opened vast reserves of natural gas in North Texas, but critics say it has caused small earthquakes.