A decommissioned Navy tugboat moored in Richmond's inner harbor was discovered to be sinking and leaking Sunday, a Coast Guard spokesman said.

The Coast Guard Incident Management Division spread a three-layer boom to contain a black discharge leaking from the vessel, according to Petty Officer Caleb Critchfield. Another boom was placed around Brook's Island just south of the inner harbor.

Critchfield said it isn't known how much fluid has leaked, how much was on board the tug, nor why the vessel began sinking.

The Coast Guard conducted an overflight of the area Sunday to assess the spill. Another overflight is expected at first light Monday morning.