A lawyer for a radical Muslim convert imprisoned for supporting terrorism urged a federal appeals court Thursday to revive the man's parental rights lawsuit, which a judge had dismissed as frivolous.

Jason LaFond told a three-judge panel of the 4th U.S. Circuit Court of Appeals that Zachary Chesser's complaint against his mother and FBI agents, while lengthy and complicated, was not fanciful and was supported by numerous documents.

The judge who dismissed the lawsuit "didn't say whether it was factually or legally frivolous, but it was neither," LaFond said.

Court papers show that Chesser attempted to join the al-Shabab terrorist group in Somalia, taking his baby with him to the airport in an attempt to look less suspicious.

It didn't work. Chesser, who had attracted law enforcement attention after posting threats against the creators of "South Park" for cartoons he felt insulted the prophet Muhammad, was arrested and pleaded guilty in 2010 to attempting to support terrorist groups and threatening violence online. He was sentenced to 25 years in prison.

His wife — Proscovia Nzabanita, the daughter of a Ugandan diplomat — was forced to leave the U.S. for lying to investigators about her husband's activities.

Chesser's mother, Barbara Chesser, won custody of the couple's baby and is raising him with her partner. The young couple "opposed placing the young boy with his grandparents, who did not share Chesser and his wife's conservative Islamic beliefs," LaFond wrote in his client's appeal.

The lawsuit says FBI agents monitoring his prison conversations tipped off Barbara Chesser, a senior lawyer in the Office of the Attorney General in the District of Columbia, that the couple was making plans with a friend to take the boy to Jordan to be raised by Nzabanita. Soon after, a judge awarded custody of the boy to Barbara Chesser.

Zachary Chesser's lawsuit seeks monetary damages, not custody. He says his privacy was invaded when FBI agents disclosed his plans to his mother.

U.S. Justice Department attorney Lowell Sturgill urged the appeals court to uphold U.S. District Judge Liam O'Grady's decision, although he conceded that the judge should be required to explain his reasoning on the privacy issue.

The court typically rules several weeks after hearing arguments.