LEWES, Del. (AP) — Crews are continuing a cleanup operation from an oil spill that affected a significant stretch of coastline in Delaware and Maryland.

The response crews have cleared oily debris and tar balls from the southern side of the Indian River Inlet in Delaware to the Assateague Island State Park in Maryland, the Delaware Department of Natural Resources and Environmental Control said this week. Beaches in Maryland are no longer affected by the spill.

The spill was detected Oct. 19 as oil washed ashore at Broadkill Beach in Delaware and was spread by tidal action. Crews have removed an estimated 75 tons of oily sand and debris from coastal areas.

“We got tons of oily debris and weathered oil off our beaches, but we’re not done yet,” DNREC Secretary Shawn Garvin said. “Our experts continue to survey our coastline, assessing the cleanup operation, and as we move ahead, conducting final evaluations of our beaches to make sure the job is done.”

The source of the spill has not been determined. The oil was described as “heavy fuel oil” likely leaking from an operating vessel, not crude oil from the hold of a tanker.

The U.S. Coast Guard has said oil samples are being analyzed by its Marine Safety Laboratory to try to find what is essentially a “petroleum fingerprint” that might help determine the source of the spill.

The Coast Guard is covering the costs of the cleanup. The agency said that if the source of the spill is identified, the responsible party will be required to reimburse the agency.

The beach in Lewes remains closed, officials in Delaware said. Officials are urging people visiting other affected beaches to stay out of the water and not walk along the wrack or high tide line.