The man who authorities say pretended to be a member of the famous Rockefeller dynasty and fabricated elaborate stories about his past will use an insanity defense when he goes on trial in the kidnapping of his 7-year-old daughter, his lawyers said Friday.

Click here for photos.

Lawyers for the man who calls himself Clark Rockefeller filed notice of the mental health defense in Suffolk Superior Court on Friday, three days after Rockefeller rejected a plea deal.

The notice does not specify what type of mental illness Rockefeller will cite at his trial, but says his lawyers plan to rely on a defense that claims he lacks "criminal responsibility" for his actions because of a "mental disease or defect" at the time of the alleged crime.

His lead attorney, Jeffrey Denner, said the illness includes "an underlying delusional state."

"Suffice it to say that there are independent experts who will testify that he was suffering from a serious and long-standing mental illness at the time of the alleged offenses in this case, and there was a causal relationship between the acts that occurred and the mental illness," Denner said.

A spokesman for Suffolk District Attorney Daniel Conley did not immediately return a call seeking comment. Assistant District Attorney David Deakin declined comment.

Rockefeller's real identity is Christian Karl Gerhartsreiter, a German national who moved to the United States in the late 1970s. Authorities say he used a string of fake identities to establish himself in wealthy circles in New York, Boston and Los Angeles.

Gerhartsreiter, 47, is accused of snatching his daughter during a supervised visit in Boston in July. After an international manhunt, father and daughter were found in Baltimore. The girl was unharmed.

The kidnapping unraveled Gerhartsreiter's mysterious past. Among the false stories he allegedly told: he was a ship captain, a physicist, a descendant of British royalty.

After investigators discovered his true identity, California authorities labeled Gerhartsreiter a "person of interest" in the 1985 disappearance and presumed slayings of a San Marino couple, Jonathan and Linda Sohus. Gerhartsreiter, who then went by the name Chris Chichester, was living in a guest house on their property when they disappeared.

His attorneys had hinted previously they would use an insanity defense in the kidnapping case.

Denner said Gerhartsreiter was distraught over a custody agreement that allowed him to see his daughter only three times a year, during visits supervised by a social worker. He is accused of kidnapping the girl during his first visit under the custody agreement.

Gerhartsreiter's former wife, Sandra Boss, won full custody of the girl when the couple divorced in December 2007.

"I think the evidence will show that he is a deeply, deeply ill individual who basically snapped ... when the pressure got too great over the loss of his daughter," Denner said Friday.

In addition to parental kidnapping, Gerhartsreiter is charged with assault and battery, assault and battery with a dangerous weapon and giving a false name to police. Authorities said he pushed a social worker who was overseeing his visit with his daughter and jumped into a waiting car. The social worker received minor injuries when he tried to grab onto the car and fell to the ground.

His trial is tentatively scheduled to begin March 23, but his lawyers said Friday they will ask to postpone it until May 11. They also said they will seek to move the trial out of the Boston area.