BP and EPA officials said Tuesday the spill apparently occurred when a malfunction allowed crude oil to enter a cooling system that draws lake water into the refinery to cool equipment and then returns that water to the lake.
Haraf said the spill area is confined to a cove area along the shoreline where BP discharges water from the refinery cooling operations into the lake.
The EPA has said the spill was not expected to pose any threat to municipal water supplies that draw on the lake's water.
Agency officials have said they are not aware of any previous oil spills at the site.
Dan Goldblatt, a spokesman for the Indiana Department of Environmental Management, said Wednesday a tentative review of recent state records also revealed no previous oil discharges into the lake from BP's Whiting complex.
Goldblatt said the state agency has no indication any oil had drifted beyond the immediate area along the lake where the spill occurred.
BP's Whiting refinery covers about 1,400 acres along the lake's shoreline.
The company completed work in late 2013 on a $4.2 billion expansion and upgrade of the refinery that will make it a top processor of heavy crude oil extracted from Canada's tar sand deposits.
That expansion sparked outrage in 2007 among environmentalists after they learned a state permit would have allowed BP to increase its discharges of ammonia and pollution called suspended solids into the lake.
BP announced in August 2007 that it would find ways to keep the expanded refinery's discharges to the limits set under its previous permits.