MEDFORD — The half brother of Jacqueline Kennedy Onassis is spending another 30 days in jail because he failed to disclose his thoughts and actions about sex as part of his court-ordered sex-offender treatment, authorities said.

James Auchincloss, 63, of Ashland, is serving three years’ probation after being sentenced in August to 30 days in jail on two felony counts of encouraging child sex abuse by having a collection of photos of naked boys at his home.

Assistant District Attorney Dave Hoppe said Auchincloss went back to jail Dec. 22 for violating the terms of his treatment, which requires that he keep a journal of his sexual thoughts and actions, and then be given a lie-detector test to verify whether his disclosures were complete.

Auchincloss admitted his disclosures had not been complete just before being given the polygraph examination, Hoppe said.

Hoppe refused to disclose details of Auchincloss’ sexual thoughts and actions. The county probation office also refused to disclose details.

“This kind of stuff does happen,” Hoppe said. “Obviously, if he had been looking at child pornography or anything of that matter, we would get him in front of a judge and send him off to prison. It’s more that he needed to comply with the sex-offender treatment program.”

Auchincloss is also prohibited from any unauthorized contact with children.

He was indicted a year ago, in 2009, on 25 counts after his longtime personal assistant, Edward McManus, told investigators he had seen Auchincloss and co-defendant Dennis Vickoren viewing pictures of naked boys in sexual poses at Auchincloss’ home.

Vickoren, 58, of Ashland, also pleaded guilty to two counts and received the same sentence.

As a boy, Auchincloss carried the wedding train of his half sister when she married John F. Kennedy.