Scientists say most of the sources contributing to a puzzling concentration of the greenhouse gas methane over the Southwestern United States appear to be natural gas production facilities.

NASA researchers said Monday they identified more than 250 sources contributing to a methane hot spot over the Four Corners region of Arizona, Colorado, New Mexico and Utah.

They include gas wells, storage tanks, pipelines and processing plants. Researchers say only a handful were natural seeps from underground formations.

They said as much as two-thirds of the methane could be from only about 25 locations.

Evidence of the hot spot dates as far back as 2003, but the origin wasn’t clear. The new study identified sources with sensors aboard aircraft in 2015.

The findings were published in the Proceedings of the National Academy of Sciences.