MINNEAPOLIS (AP) - The U.S. Coast Guard says the alleged environmental violation that has stranded a German-operated cargo ship for about a month outside the Lake Superior port of Duluth involved the discharge of oily water.

The Coast Guard also said Thursday night that it's negotiating a security agreement that would allow the oceangoing Cornelia to depart while protecting the integrity of the investigation and the interests of the vessel's crew.

The statement says the discharge apparently didn't happen in the port of Duluth-Superior but doesn't elaborate.

A Coast Guard spokesman said Friday that he can't provide further details about the discharge or what the security agreement would entail.

The U.S. attorney's office in Minnesota is conducting a criminal investigation.