“Who would steal a 90-pound tortoise?” an anchor on WABC-TV in New York said on its newscast one day after the theft. “That is the question the police in Queens are trying to get to the bottom of today.”

Back in Connecticut, Millenium’s new owner started to see all the news coverage about a missing 17-year-old tortoise in Queens. Believing he might have stolen goods, the man called the police.

It did not take long for a detective to get to the bottom of the case. The detective called the number of the man who responded to the Craigslist advertisement, and Mr. Waters answered, the authorities said. Mr. Waters told the officer he had traveled to Connecticut with the tortoise to trade him for a musk turtle.

Millenium was returned to his home unharmed.

Mr. Waters was charged with fifth-degree criminal possession, a misdemeanor, and he pleaded guilty last month. In the days leading up to his sentencing hearing on Tuesday, his mother said, her son tried to make the most of his free time. He recently spent a day exploring sites in Manhattan with his 16-year-old daughter and has been telling his mother not to worry about him when he is in jail.

On Thursday morning, Ms. Waters drove her son to court in Queens, but she said she was too upset about the whole ordeal to be in the courtroom for the sentencing. “I know that Shawn is going to be the laughingstock,” Ms. Waters said.

A judge sentenced him to six months in jail at Rikers Island. Mr. Waters, who lost his job and license as a security guard after his arrest, hopes to get his license back when he is released, his mother said.