MIAMI — When the mother of a military veteran arrested and detained in a dangerous foreign jail called her congresswoman’s office two weeks ago asking for help, Rep. Ileana Ros-Lehtinen said she had a hard time believing former Marine Lance Cpl. Jon Hammar was in prison for carrying a six-decade-old shotgun into Mexico.

“We said, ‘Surely she must be exaggerating,’ ” recalled Ros-Lehtinen, a Miami Republican.

She wasn’t. Olivia Hammar’s son had been in a state prison in Matamoros, along the Mexican border, for nearly four months.

His parents, who live in Palmetto Bay, tried to resolve the matter quietly, hiring attorneys in Mexico and dealing with the U.S. consulate there. But the effort went nowhere.

Now the family has gone public — so public that even Hammar’s jailors have seen the case in the news.

“What’s going on?” Hammar asked in a late-night phone call Wednesday, according to his parents. “The guards are going crazy down here.”

The story of how Hammar, a 27-year-old who returned from the Iraq and Afghanistan wars with post-traumatic stress disorder and hoped to find peace surfing in Central America, wound up chained to a bed in Mexican prison drew widespread attention after McClatchy, The Miami Herald’s parent company, published a report about the case last week.

In August, Hammar and another ex-Marine who had suffered from PTSD crossed into Mexico in a 1972 Winnebago motor home, planning to drive to Costa Rica. They were detained because Hammar had brought a .410-bore Sears & Roebuck shotgun that once belonged to his great-grandfather.

Hammar had been instructed by U.S. authorities to fill out registration papers for the shotgun, suitable to shoot birds, his family said. But Mexican authorities dismissed the paperwork and charged Hammar with possession of a weapon restricted for use by Mexico’s armed forces — a serious crime. A conviction could carry a sentence of three to 12 years.

Days after Hammar arrived in prison, his parents received phone calls from other inmates involved with a drug cartel demanding extortion money, the Hammars said.

They didn’t pay. U.S. diplomats got Hammar pulled out of the general inmate population and into solitary confinement. There, Mexican officials have said that Hammar is periodically chained to a bed or a wall to prevent him from attempting to flee, Ros-Lehtinen told reporters in her Miami office Thursday morning.

Ros-Lehtinen and Democratic Florida Sen. Bill Nelson have spoken about Hammar on the floor of the U.S. Capitol. More than two-dozen members of Congress have signed a letter to Secretary of State Hillary Clinton and Homeland Security Secretary Janet Napolitano drafted by Ros-Lehtinen’s office.

“I’m optimistic that everything’s going to work out,” said Ros-Lehtinen, who was joined by Miami-Dade Commissioner Jose “Pepe” Diaz, himself a former Marine, and other elected officials. “In these few days before Christmas, we’re going to bring Jon Hammar home.”

Hammar’s friends from his alma mater, Westminster Christian School, and from his church, Old Cutler Presbyterian, both in Palmetto Bay, have also rallied, together with his younger sister Katie’s swimming buddies. They have created a pair of online petitions asking for Hammar’s release. As of Thursday afternoon, the two petitions had garnered more than 12,000 signatures.

More than three-dozen of Hammar’s friends gathered at Ros-Lehtinen’s office Thursday, wearing yellow ribbons to support military families. Classmates of Hammar’s sister, who has not been able to come home yet because she’s taking college exams in North Carolina, said they have decorated trees at Westminster with messages for Hammar.

“Everybody loves him,” 22-year-old Brittany Hays said of Hammar, whom she has known for more than a decade.

Several friends called Hammar kind-hearted and easygoing, with a surfer’s long locks and laidback demeanor. As a kid, Hispanic friends nicknamed him Juan Martillo — a play on the phonetic sound of his name, “John Hammer.” In high school, a string of girls had crushes on him. He loved the water and volunteered teaching disabled children how to sail, a high school classmate and neighbor said.

After the 9/11 terrorist attacks, Hammar and a group of friends enlisted in the Marine Corps. He was deployed to Afghanistan and Iraq, where his battalion lost more than a dozen troops, before receiving an honorable discharge in 2007. He later served four more years in inactive reserve.

Last year, Hammar voluntarily checked into a residential treatment center for veterans in California, where he was treated for nine months for PTSD. (His mother said he complained that the California surf was too cold. “I’ve got to move back to Miami,” he told her.)

In prison, Hammar has reverted to “combat mode,” said his mother, who calls her son Jonny.

“In addition to being somewhat hopeless, he is enraged,” she said. “He’s read the entire Bible, because that’s all he has.”

Olivia Hammar, a magazine publisher, said she Googles news about Mexico every morning to find out of anything violent has happened at or around the Matamoros prison. She expected Hammar to return from Mexico in September, when her husband, a software engineer also named Jon, traveled to Matamoros to fight for his son’s release.

“It’s a precarious situation now that we’ve gone public,” Jon Hammar said, calling for an end “to this silly, unfortunate thing that is happening.”

Hammar’s friends, hoping for his return by Christmas, are planning a celebration on Dec. 26. Also in the works: a plan to raise money to send Hammar to Costa Rica — on a plane.

“We’re just so proud of everything that he’s done,” friend Jessica Fernandez said. “We’re not going to rest until he comes home.”