But after he drove his Winnebago a few feet farther to the Mexican side, authorities arrested him, saying the weapon did not comply with their gun laws.

"The crime that he's charged with is possession of a weapon that's restricted for military use," said his mother, Olivia Hammar.

A branch of the Mexican military has said the gun is not on its "forbidden list," she said. But the former Marine remains incarcerated.

Mexican officials said that the files on the case were not immediately available because the arrest happened so long ago. An official at the prosecutor's office declined to comment.

A few nights after Hammar's arrest, his parents received the first of several threatening calls from behind bars, they said.

"He said: I have your son," Olivia Hammar recalled, tearing up. "I am going to f--- him up. I already have."

Then she heard her son's voice.

"He said: Mom, you've got to do what they say; they're really serious."

The voice at the other end of the line asked for $1,800.

U.S. Sen. Bill Nelson, a Democrat from Florida, stepped in on Hammar's behalf, speaking to Mexico's ambassador to the United States.