Ex-Marine Jon Hammar held in Mexico on gun charges MIAMI, Dec. 12 (UPI) -- The family of a former U.S. Marine arrested in Mexico on weapons charges, which the family says have been proven false, is calling for his release. WPLG/ABC News in Miami reported Wednesday former Marine Jon Hammar, who, along with fellow serviceman Ian McDonough, had recently completed a treatment program of post-traumatic stress disorder after fighting in Afghanistan, embarked with McDonough on a driving trip to Costa Rica. On Aug. 15 the pair arrived at the border between Mexico and Texas. Hammar had brought along his great grandfather's antique shotgun. Border officials arrested the two on federal charges of possessing a weapon reserved for military use. Hammar said the gun was his and McDonough was released. Olivia and Jon Hammar Sr. said Jon was taken to a state prison in Matamoros, Mexico. They received calls from their son that made them realize he was in danger from other prisoners who used him to try to extort money from his parents. 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. He said, 'You need to do whatever they say. I'm so sorry. I'll pay you back," Olivia Hammar said. After the parents contacted the U.S. consulate in Matamoros, their son was held in isolation and relative safety, WPLG/ABC News reported. 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. 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, Hammar's Mexico City attorney, said. Levy said he hopes to have Hammar released from jail within the next month. He said he is working with prosecutors to discuss evidence and possible lesser charges in the case.