A giant oil slick from a deadly offshore drilling rig explosion could hit the fragile U.S. Gulf Coast shoreline this weekend as the White House and Congress launched separate probes into the worst offshore incident in nearly a decade.

The leaking well, 5,000 feet under the ocean surface off Louisiana’s coast, has created an oil sheen and emulsified crude slick with a circumference of about 600 miles, covering about 28,600 square miles (74,070 sq. km), the Coast Guard said on Tuesday. That’s slightly bigger than the U.S. state of West Virginia.

Swiss-based Transocean Ltd’s Deepwater Horizon sank on April 22, two days after it exploded and caught fire while finishing a well for BP Plc about 40 miles southeast of the mouth of the Mississippi River.