AUSTIN, Texas (AP) — The regulatory agency overseeing Texas' oil and gas industry has determined that a series of small earthquakes in North Texas likely wasn't caused by drilling operations by an Exxon Mobil subsidiary.

The preliminary findings mark the first decision by the Texas Railroad Commission since it was authorized last year to consider whether seismological activity was caused by injection wells, which store briny wastewater from hydraulic fracturing.

The commission ordered hearings after a university study suggested two companies' wells were responsible for quakes that shook Reno, Texas, in 2013 and 2014.

Commission investigators concluded that a well where Exxon Mobil subsidiary XTO Energy pumps millions of gallons of the wastewater likely didn't cause the quakes, but also said there wasn't enough evidence to demonstrate the earthquakes were naturally occurring. Parties have 15 days to respond.

The report was released Monday, a day before a new law took effect barring Texas cities and towns from banning hydraulic fracturing, or fracking, and limiting local authority to restrict other oil and gas operations.

Reno Mayor Lynda Stokes, whose city passed an ordinance banning injection wells, said she wasn't surprised by the commission's findings and wasn't sure what legal recourse was available.

"They've pretty much given industry free reign to roll over us," she told The Associated Press.

The study published by Southern Methodist University geologists in April pinned the earthquakes to an XTO well and a well operated by Houston-based Enervest.

The study concluded that the quakes were triggered by rumblings in the shallow Ellenburger formation that migrated down the fault into the deepest layer of rock. The companies denied any connection.