CHICAGO (CBS) — Crews at the massive BP refinery in Northwest Indiana are mopping up after the discovery of a small amount of oil near a pipeline valve.

According to a BP spokesman, officials are investigating whether the oil leaked from the pipeline or is from another “existing or historic source.”

The oil was discovered during a routine inspection on Tuesday on a pipeline that supplies oil to the refinery in Whiting, Ind.

“BP continues to monitor the pipeline closely and will make repairs and clean up as needed,” the spokesman said.