Only days after U.S. Marine veteran Jon Hammar was thrown into a Mexican prison for carrying an antique shotgun into the country, gangsters in the jail warned him of his likely fate — beheading.

“They threw every threat in the book at me,” Hammar said Thursday in his first public remarks since his release Dec. 21 after more than four months in prison.

“They’d cut my head off, they told my family,” he said.

The gangsters demanded money to let Hammar, 27, remain alive, and the beheading threat was a scare tactic that harkened back to his tours of duty as a U.S. Marine in Iraq.

Mexican authorities arrested Hammar on Aug. 13 at the Texas-Mexico border when he crossed into Mexico in a motor home with a vintage shotgun that once belonged to his great-grandfather. Authorities slapped weapons charges on him. A traveling companion went free.

Hammar’s ordeal, first brought to light in a McClatchy story on Dec. 6, sparked outrage in the United States, where fellow Marines demanded his release and members of Congress called for a boycott of Mexican tourism.

The outpouring has left Hammar feeling grateful.

“In America, we have people who care,” he said in a telephone interview from his family home in Palmetto Bay.

“I’m really grateful. But at the same time I kind of expected that from Marines,” he said. “Marines don’t just throw each other under the bus. They look after each other.”

Despite his release just days before Christmas, Hammar only now is recovering from a stomach virus and dehydration that required hospitalization that he blames on the conditions he encountered during his incarceration.

When his father received him at the Texas border, the two began the 22-hour drive to Florida to join his mother for the holidays.

Once they got to Lafayette, La., Jon Hammar began developing a fever, and they spent much of a day in a hospital there. He was hospitalized further shortly after arriving in South Florida.

“I think it was just an accumulation, just not being in a good environment,” Hammar said.

After Hammar’s parents complained to the U.S. Consulate in Matamoros of the gangster threats against their son, the younger Hammar was moved out of the general prison population to a loosely guarded cage, where he spent months with his ankle handcuffed to a bed frame.

Once physically separated from the gangster inmates, the threatening phone calls to Hammar’s parents stopped, he said.

The prison where Hammar was kept in Matamoros is notorious for being under the control of the dreaded Los Zetas criminal gang, which has used beheadings and mass killings to instill fear in rivals and protect their drug trafficking and criminal activities.

Hammar said prison inmates controlled the facility, not the guards.

“There was a tattoo artist in there, and he said to me, ‘You can die in this place for nothing,’ ” Hammar said.

Prisoners are “full-blown” mobsters, he said, some of them serving up to triple life sentences.

“I knew not to go around picking fights with anybody,” he said.

Compared to prison guards and judicial employees, however, the gangsters struck Hammar as efficient.

“To me, they were acting more professional than the officials in Mexico. If I needed a translator, and the cartel was talking to me, I had three. But when I’d go to court, they laughed at me when I asked for a translator,” he said.