Dec. 12, 2012 -- A former U.S. Marine who took off on a surfing adventure to Costa Rica in August is stuck in a Mexican jail just over the border from Texas, and his family is calling for his release.

Ex-Marine Jon Hammar headed south with fellow veteran Ian McDonough on what was supposed to be a few months of surfing and camping in a Winnebago in Costa Rica. The two had recently finished a treatment program for post-traumatic stress disorder, which Hammar suffered after fighting in Fallujah, Afghanistan, according to his mother, Olivia Hammar.

"The treatment's very exhausting, it's a tough program, and he was there almost nine months," said Olivia Hammar. "(They) decided they were going to buy an R.V., fix it up, drive down to Costa Rica through Mexico, and we were very nervous about it. We tried to discourage it, to tell him to take a plane, but they said, 'We're taking nine surfboards and need a place to stay.'"

Hammar and McDonough arrived on the border between Mexico and Texas on August 15. Hammar, however, had packed his great grandfather's shotgun, a .410 Sears and Roebuck model nearly 100 years old. Hammar had hoped to hunt small birds with it while living in Costa Rica, Olivia said. The pair wanted to register the gun with Mexican authorities at the crossing point.

"There were signs that said you can't take a firearm, and so Ian said scrap it, don't take it, but Johnny said, 'Let's talk to the customs agent,'" according to Olivia. "They said, 'Technically you can (bring it across) but you'll need to register it,' and had (Johnny) fill out paperwork to present to Mexican officials."

The gun was meant for hunting, but border officials arrested the pair on federal charges of having a weapon that is reserved for military use. McDonough was released when Hammar claimed the gun was his.

Olivia and Jon Hammar, Sr., hired local lawyers to defend their son in Matamoros, Mexico, where Hammar was taken to state prison. The U.S. State Department was notified by Mexican authorities the following day, according to a department official who spoke on background.

But once Hammar was in prison, his family said they began receiving irregular phone calls from Hammar, sometimes in the middle of the night, and sometimes accompanied by other prisoners demanding money.

"Almost immediately we began receiving extortion calls from cartel members in prison with him," Olivia said. The State Department and Hammar's lawyer, Eddie Varon Levy, would not comment on the claim about cartel members.

"They're saying, 'You need to wire us money or we're going to kill your son, we've already f---ed him up,' and initially I thought it was a scam, but then they put him on the phone and he was breathless and I knew they had," Olivia said. "He said, 'You need to do whatever they say. I'm so sorry. I'll pay you back.'"

Olivia and Jon Sr. say that, filled with panic, they contacted the U.S. consulate in Matamoros, Mexico, which arranged to have Hammar isolated from the general prison population. They were advised not to pay any ransom money, Olivia Hammar said.

A State Department official said, "The safety and well being of Mr. Hammar is a serious matter.... We requested he be moved away from the general prison population, and prison authorities granted that request. Now, he is in a separate room with constant contact with prison personnel."

Olivia Hammar said that at one point, her son was chained to his bed in a small storage shed. The State Department told ABC News that they have since rectified the situation and Hammar is no longer chained.

"He was really hopeless. There have been times when I've talked to him and he's angry and said, 'I'm fine,' and then you'll talk to him again and he'll say, 'I can't take another minute of this,'" Olivia said tearfully.

Hammar had been a lifelong surfer and sailor who loved being outdoors, his mother said. He enlisted in the Marines at age 18, in 2003, to challenge himself. When he returned from his second tour of duty in Afghanistan in 2007, after his unit lost 16 soldiers, he was "a different man," she said.

"He's such a free spirit," she said. "He was the kid I could never keep inside and the thought of him being chained...."

Now, the Hammars are hoping that Varon Levy can help extricate their son from the Mexican judicial system. Varon Levy, speaking with ABC News from Mexico City, said that the charges Hammar was initially arrested on proved false; he was not carrying a banned weapon that was only for the military. The actual criminal charges were brought because the barrel of his shotgun was too short.

"The gun is not for the reserved use of the army, and the bullets were not for the reserved use of army, but the barrel falls 18mm below what the barrel should be," Varon Levy said. "This is a Mexican law of weapons and explosives that was violated. You can take a gun for hunting, that's not the problem. The problem is whether the barrel was measured wrong."

Varon Levy said he hopes to get Hammer out of jail within the next month, as he works with prosecutors to discuss evidence, witnesses, and possible lesser charges in the case.

"We're not getting anywhere, they just keep postponing," Olivia Hammar said Tuesday. "At this point we're not sure which is worse, the cartels or the government.

But Varon Levy said the case has taken the appropriate amount of time.

"This is a trial just like in the U.S.," Varon Levy said.

Calls to the Mexican Embassy in the U.S. were not immediately returned.