The U.S. Coast Guard responded to the report of an oil sheen near a bulk carrier anchored in Baltimore’s Inner Harbor Thursday night.

Watchstanders from the U.S. Coast Guard Sector Maryland National Capital Region received a report around 6:42 p.m. about an unknown sheen around the SSI Defiant following what the Coast Coast described as a “propulsion casualty,” according to a news release.

A 1,500-foot safety zone was established around the vessel, as Coast Guard crews, assisted by the Maryland Department of the Environment and other agencies, conducted assessments and investigated aboard the ship. The cause and nature of the 50-foot by 50-foot sheen was unknown, the release said.

The Coast Guard monitored the vessel’s pollution response to ensure sufficient environmental protection. According to the website VesselFinder, the SSI Defiant is a bulk carrier that transports unpackaged raw materials. It was built in 2013 and operates under the Liberian flag. The vessel was last reported leaving a port in Spain on Sept. 13 before arriving in Baltimore, according to VesselFinder.

Overflights of the area were conducted, according to the release, and additional overflights were scheduled. The investigation of the sheen’s cause is ongoing.