Mexican President Enrique Peña Nieto has the power to pardon former Marine Jon Hammar if the veteran is convicted – but it’s unclear whether Mexico’s highest leader would do so.

Hammar’s fate now rest in the hands of a judge – who will determine whether he is guilty or innocent. If Hammar is convicted of federal charges, his future then lies in the hands of Peña Nieto. The president's office did not return phone calls seeking comment.

But experts say don't expect a presidential pardon.

Freeing Hammar could undermine ongoing efforts made by both Peña Nieto and the U.S. to modernize Mexico's justice system so that everyone can be treated the same "avoiding deals that only the rich and powerful can make", according to Diana Negroponte, a senior fellow at the Brookings Institute.

"I would not expect the president to get involved, unless there was some other political exchange to be made," Negroponte said.  "Peña Nieto will insist on transparency, putting into practice the new judicial system under his reforms and making a real effort to move away from the old inquisitorial Spanish system."

Afterall, Negroponte said, "Do we want to have powerful Mexican politicians calling up and interfering with our justice system?"

While Hammar's future is uncertain, what is clear is this: Hammar will not be home for Christmas.

Hammar, who served two tours of duty in Iraq and Afghanistan, "will remain in detention for the duration of his trial" in Mexico where he faces a felony charge and up to 15 years in one of Mexico's most dangerous prisons, according to a letter written by Mexican Ambassador Arturo Sarukhan.

Hammar was arrested on federal charges on the border for entering Mexico on August 13 for possessing his grandfather's antique .410 shotgun, used to kill rabbits and birds, because it was a weapon reserved for military use in Mexico, according to a letter dated December 12, from the ambassador to U.S. Rep. Illeana Ros-Lehtinen (R-Fla). In the letter, the ambassador also notes Hammar's military service but reiterates Mexico's "very stringent gun control laws" particularly "as a result of the weapons illicitly purchased in the U.S. and then trafficked into Mexico."

The letter is a blow to Hammar's lawyer Eddie Varon-Levy and his family who had hoped the case could have been resolved before Hammar's next court date on January 17. Hammar remains in solitary confinement with his right ankle chained to a bed in the administrative offices of the jail in Matamoros, a notoriously dangerous state prison.

"It's devastating. It's incomprehensible, a lot of what he says in that letter is simply not true," Olivia Hammar, the marine's mother, said. "My son, a marine who served two tours of duty, can get a felony and not be able to vote in this country again." 

The 27-year-old and his marine friend, Ian Mcdonough, were arrested by Mexican agents on federal charges of possessing a weapon reserved for military use while traveling to Costa Rica in a refurbished 1972 Winnebago motor home. Hammar and his family said the gun, which he had declared to U.S. border agents, was his great grandfather's antique shotgun. The U.S. agents then told him to declare the gun with Mexican agents. Hammar did and subsequently both he and his friend Mcdonough were arrested by Mexican custom officials. Mcdonough was let go a few days later because he did not own the gun.

"The fact that he declared to Mexican Customs that he was in possession of the weapon does not preclude arrest and prosecution," the ambassador wrote in the letter given to the Hammar family on Monday.  

Regardless of what was declared, the ambassador said, "the sole introduction or possession of this type of weapon (not withstanding its intended use or year of manufacture) constitutes a federal crime in Mexico and is not subject to any prosecutorial discretion."

The Hammar family has denied that the weapon is illegal and point to a letter sent to them from the Secretary of Defense in Mexico dated August 24 that assured the Hammar family the antique shotgun is not a weapon used by the Mexican military.

"Johnny made an administrative mistake. He should have registered the gun in an embassy," the mother said. "He's been detained for months because of a clerical error."

The ambassador's letter also implied that the Hammar family had a chance to plead guilty to a lesser charge, "which would have in all likelihood resulted in his release and repatriation in the United States."

But Hammar said that was simply not true.

"If we ever had a chance to take my son out of that jail months ago, we would have done it," Hammar said. 

Ambassador Sarukhan also defended the prison's decision to chain the marine and his placement in solitary confinement in an administrative office as a "security measure" to "ensure his safety" from other inmates, many of whom are cartel members.

"The lack of cells in this area - or of any security measures there - left the authorities with no other choice but to temporarily restrain or limit his movement," the letter said.

The ambassador also suggested that the family had a chance to transfer Hammar to a federal prison "where better conditions would ensure his safety and well being," but said his legal counsel rejected the offer. The ambassador also pointed out that the U.S. Consular officers in Tamaulipas state "have regularly visited him and assisted him."

Olivia denies that they have had a chance to move her son to better conditions. But she told Fox News Latino they did specifically ask the government not to transfer Hammar to another jail because the prison system was going through a series of violent, sometimes deadly jail outbreaks – and they did not want their son caught up in that.

Rep. Ros-Lehtinen (R-Fla) spoke on the floor of the House of Representatives Monday night pleading Congress to act to free Hammar.

"Right now, aside from assuring Jon's immediate return, my main focus is Jon's health and well-being," Ros Lehtinen said in a statement to Fox News Latino. "I want to make sure that Jon is being treated humanely, and has proper access to water, food, and most importantly, access to the medical attention someone who suffers from post-traumatic stress disorder requires. I have reiterated these concerns in a letter to President [Enrique] Peña Nieto, and I trust he will do the right thing and see to it that this matter is resolved as quickly as possible."

The family is pleading with the new Mexican president to act on the marine's behalf. According to the Mexican Constitution, if Hammar is convicted of a federal crime, Peña Nieto has the power to pardon him.

The ambassador's letter assured the family and the U.S. government that "the government of Mexico is taking this situation very seriously."

"We all seek and strive for expedited justice and a fair resolution of this case. However, the fact that this case has been in process since August is normal given the nature of the felony he has been detained for," the ambassador said, referring to the alleged crime as a "serious offense."

The letter ends with a promise from the ambassador that he will "personally continue to stay engaged on this issue."

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Bryan Llenas currently serves as a New York-based correspondent for Fox News Channel (FNC) and a reporter for Fox News Latino (FNL). Click here for more information on Bryan Llenas. Follow him on Twitter @BryanLlenas.

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