An ExxonMobil pipeline running under the Yellowstone River in south central Montana ruptured late Friday, spilling crude oil into the river and forcing evacuations.

The pipeline burst about 10 miles west of Billings, coating parts of the Yellowstone River that run past Laurel — a town of about 6,500 people downstream from the rupture — with shiny patches of oil. Precisely how much oil leaked into the river was still unclear. But throughout the day Saturday, cleanup crews in Laurel worked to lessen the impact of the spill, laying down absorbent sheets along the banks of the river to mop up some of the escaped oil, and measuring fumes to determine the health threat.

Fearing a possible explosion, officials in Laurel evacuated about 140 people on Saturday just after midnight, then allowed them to return at 4 a.m. after tests showed fumes from the leaked oil had dissipated, The Associated Press reported. While the cause of the rupture was not immediately known, Brent Peters, the fire chief for Laurel, told The A.P. that it may have been caused by high waters eroding parts of the river bed and exposing the pipeline to debris.