State Sen. Nathaniel Oaks, indicted earlier this year for charges including wire fraud, agreed to cooperate with the FBI on a new investigation then allegedly tipped the target off to the ongoing investigation.

Federal prosecutors say a federal grand jury returned a superseding indictment also charging the 71-year-old Democrat from Baltimore with obstruction of justice. His trial is set for April 16.

Prosecutors say Oaks agreed to cooperate with the FBI in January. He covertly recorded his telephone conversations and meetings with the target from then until March 30. Prosecutors say Oaks knew that he could be made to testify about those interactions. They didn't go into detail on the nature of the investigation.

On March 17, without recording or telling FBI agents about the conversation, he allegedly approached the target at an Annapolis bar and told him, "What we talked about, just say no." On March 30, he again approached him in a state government building and allegedly coached the target: "I'm going to ask you for something, just say no."

The target took those statements to mean that there was a criminal investigation underway, prosecutors say.

Under the existing charges, Oaks already could spend the rest of his life in prison if convicted. Obstruction of justice would carry an additional 20 years. An initial appearance has not yet been set.

Oaks first came to Annapolis as a state delegate from District 41 in 1994. He was appointed to fill the same district's state Senate vacancy in February.