The British company said about 39 barrels of oil spilled from its Whiting refinery in Indiana into Lake Michigan. It blamed the incident on a malfunction from a distillation unit at one of the country's oldest refineries.
U.S. Sens. Mark Kirk, R-Ill, and Dick Durbin, D-Ill., said they had questions about BP's operations given that Whiting was expanded recently to process more of the heavy grade of Canadian crude oil.
In a letter to BP America Chief Executive Officer John Minge, they said they wanted assurances Whiting's operations were safe.
"We urge you to explore every avenue to expeditiously recover any spilled oil, remediate the damage where possible, and minimize future threats the Whiting Refinery poses to [Lake Michigan]," their Thursday letter reads.
A 2010 spill of Canadian crude oil in southern Michigan was one of the worst onshore incidents of its kind. The heavier grade, dubbed oil sands, sinks in water and makes it more difficult to recover.
The U.S. Environmental Protection Agency said this week most of the Lake Michigan shoreline was clear of oil.