ANCHORAGE, Alaska (AP) — Alaska regulators are investigating after crude oil was discovered along a buried section of the trans-Alaska pipeline north of Fairbanks.

Alyeska Pipeline Service Co. has recovered 10 gallons (39 liters) of crude oil discovered Sunday following an excavation at the site south of Atigun Pass, the Anchorage Daily News reported .

The oil was discovered in the soil as crews were inspecting a mainline valve, said Michelle Egan, an Alyeska Pipeline spokeswoman. An excavation at the site is continuing, she said Tuesday.

"Engineering and field personnel are assessing the situation and developing plans to safely excavate the valve and to determine the cause of crude oil in the excavated area," Egan said.

The oil could have been overlooked from the 2013 oil release, which occurred at the same site, said Ashley Adamczak, an environmental program specialist with the state Department of Environmental Conservation. That release of 21 gallons (79 liters) was linked to soil movement that occurs as tundra freezes and thaws, she said.

Inspection crews had expected to find some trace of oil left from that release, but the amount found was more than expected, Adamczak said.

"We're working through Alyeska to develop a cleanup plan, and to figure out if this is contamination left from the 2013 release or whether it is indicative of a new issue at the valve," Adamczak said.

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Information from: Anchorage Daily News, http://www.adn.com