Miami (CNN) -- For a war vet from South Florida, a leisurely road trip to a seaside vacation came to an abrupt end in Mexico, where he has languished behind bars for four months, sometimes chained to a bed.
For the first few months, Jon Hammar's parents kept his plight out of the public eye, fearing that media attention could bring harm to him in prison.
But now they are coming forward, hoping the publicity will push Mexican authorities to act.
Hammar, 27, is being held in the border town of Matamoros, just across from Brownsville, Texas.
It is notorious as the home of the Gulf Cartel, one of Mexico's major drug trafficking organizations.
"The only reason they were going to stop was to get more gas," his father said.
Hammar, who served in Iraq and Afghanistan as a Marine, was headed to Costa Rica in August to go surfing with a fellow veteran.
He had surfed there before and packed every good surf board he had, his parents said.
But he also took one item that quickly became troublesome: an antique shotgun handed down from his great-grandfather.
He intended to hunt with it, his parents said. And he declared it with U.S. border agents.
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.
"We interceded to get him out of the general prison population," Nelson said. This served to remove Hammar from contact with dangerous inmates.
The calls have since stopped, his mother said.
"He is now being kept in a low ... intensity place, more like an administrative place," Nelson said.
But because of the low security of the new facility, which Olivia Hammar describes as a storage shed, officials periodically chain Hammar to his bed.
The Hammars recently turned to their U.S. congresswoman, Ileana Ros-Lehtinen, who heads the Committee on Foreign Affairs.
"This is outrageous," Ros-Lehtinen said. She has spoken with the State Department and the U.S. ambassador to Mexico, and will meet with the Mexican ambassador to the United States this week. "Enough is enough," she said.
"Jonny, we're going to get you out of there," Olivia Hammar said to her son by phone Friday.
He reminded her that she has been telling him that since August.
Their son returned to them safe and unharmed from Iraq and Afghanistan. Hammar's parents now fear they could lose him just across the border, if they wait too much longer.
CNN's Gary Tuchman reported from Miami and Ben Brumfield wrote from Atlanta.