By Joseph Ax

NEW YORK (Reuters) - A U.S. appeals court on Tuesday appeared sharply divided on whether to reinstate the conviction of New York's so-called "cannibal cop," who was found guilty of plotting to kidnap and devour women and then set free when a judge overturned the jury's verdict.

Former New York City police officer Gilberto Valle was convicted in March 2013, based largely on his exchanges with others on a dark fetish website about his desire to torture and kill several women he knew, including his wife.

Prosecutors said Valle used a law enforcement database to gather information on women he planned to abduct, torture, cook and consume.

But U.S. District Judge Paul Gardephe last year ordered him released, agreeing with Valle's lawyers that the gruesome descriptions were merely fantasy, not part of an actual criminal plot. None of the women were ever harmed.

The office of Manhattan U.S. Attorney Preet Bharara on Tuesday urged the 2nd U.S. Circuit Court of Appeals to send Valle back to prison, arguing that he took other steps such as researching chloroform recipes.

"It is not Judge Gardephe's role to seat himself as a 13th juror," Assistant U.S. Attorney Justin Anderson told a three-judge panel.

Edward Zas, Valle's lawyer, said the messages were merely "horrible fantasies" and that Valle often made seemingly concrete plans with others online that never came to fruition.

"In an ordinary case, words are pretty good evidence of intent," he said, but "not in this case."

U.S. Circuit Judge Barrington Parker appeared skeptical of the government's argument, comparing Valle's supposed plans with a drug conspiracy in which a leprechaun transports cocaine from the moon.

U.S. Circuit Judge Chester Straub questioned the defense argument by pointing out that Valle traveled to Maryland to have lunch with one woman and also learned where she worked.

The third judge, Susan Carney, expressed doubt about whether a real conspiracy existed, although her questioning was less sharp than that of her colleagues.

A decision is not expected for some time.

Valle's wife turned him in after discovering disturbing images and emails on his computer.

The case also led to the arrest of three other men for conspiring to kidnap and kill women.

One of them, Richard Meltz, pleaded guilty and was sentenced to 10 years in prison. Christopher Asch and Michael Van Hise were convicted last year and are appealing.

(Reporting by Joseph Ax; Editing by Barbara Goldberg and Peter Cooney)