A Canadian man who repeatedly entered Michigan to buy and ship thousands of turtles to his native China was sentenced to nearly five years in federal prison Tuesday for smuggling.

It was a tough punishment for Kai Xu, who has been locked up for 19 months since his arrest and had hoped to be released. The 27-year-old expressed remorse to a judge and thanked agents "for stopping the darkness of my greed and ignorance."

Ahead of the hearing, Xu wrote a letter to U.S. District Judge John Corbett O'Meara, saying he sold turtles partly to make money for college. He said he was a semester short of an engineering degree from the University of Waterloo in Waterloo, Ontario.

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The government said Xu shipped turtles to China from Canada and the U.S., or hired people to fly to China with turtles in their luggage. In 2014, he was caught at the Ontario, Canada, border with 51 turtles taped to his legs.

It's not illegal to buy turtles from breeders in the U.S., but Xu's crime was shipping them overseas without a federal permit.

Xu was not a "sophisticated international dealer," defence attorney Matthew Borgula told the judge, adding that hiding turtles under his pants was "not a good way to get them across the border."

Assistant U.S. Attorney Sara Woodward said Xu's remorse was genuine, but described his smuggling scheme as one of the largest in recent years.

Woodward asked for five years in prison, near the low end of the sentencing guidelines. O'Meara settled on 57 months without an explanation, though he praised Xu for becoming fluent in Spanish while in prison and helping Hispanic inmates.

"We don't have a whole lot of cases exactly like this every day," O'Meara said of turtle smuggling.

The guidelines were enhanced by the value of the reptiles as estimated by the government. Prosecutors said shipments intercepted at airports were worth more than $1 million.

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Borgula objected to the government's conclusion and asked that O'Meara hear testimony. The judge declined, saying he was pressed for time. An appeal is planned.

"The sentence is very severe," Borgula told The Associated Press.