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The mother of a U.S. Marine being held in a Mexican jail after he crossed the border with guns in his pickup truck said her son’s current ordeal is more traumatic than the two tours of duty he served in Afghanistan.

Andrew Tahmooressi, 25, faces up to 21 years in prison and has already lost more than 20 pounds since being arrested March 31 at a border crossing near San Diego, according to his mother, Jill Tahmooressi. She is frightened for her son, who she said suffers from post-traumatic stress disorder and had three guns in his pickup truck because he is in constant fear for his safety.

"We have a decorated Marine being held in a Mexican prison for no reason," said Jill Tahmooressi, of Weston, Fla. "By far, this is worse than Afghanistan. At least he was in Afghanistan by choice, proud and honored. Now he is being held captive under inhumane conditions.”

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Andrew Tahmooressi was arrested by Mexican military after border inspectors found three legally purchased and registered weapons in his truck. Although he claims he made a wrong turn at the poorly-marked crossing, he was taken to the notorious La Mesa State Prison in Tijuana.

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Tahmooressi did not even realize he was in Mexico because of the poorly lit area and a small sign covered in graffiti, conditions verified by Fox News Channel’s Greta Van Susteren, who retraced Tahmooressi’s path for a segment that aired on Monday night. He entered Mexico at a substation border crossing and not the main San Ysidro border crossing. Once Tahmooressi found himself headed to the station, there was no opportunity to turn around.

"He was in the town of San Ysidro and you think you're getting onto I-5 but you're actually driving to a point of no return,” said the Marine’s mother.

Death threats at La Mesa prompted Tahmooressi to attempt an escape, an effort that got him shackled in his prison cell under deplorable conditions for more than a month, his mother said. He has since been transferred to El Hongo Federal Penitentiary, where he remains.

“All they feed him for dinner is bread and sugar water, which he discards, and a protein source for lunch and breakfast," said Jill Tahmooressi, who is allowed to speak with her son daily by phone.

Tahmooressi served two tours of duty in Afghanistan between 2010 and 2012, and he received a battlefield promotion to sergeant because of his leadership and heroism in the face of fire in the notorious Helmand Province. But once he was back home in Florida, where he was plagued by PTSD, a fellow Marine suggested he go to California for treatment at the Veterans Administration facility in La Jolla. He was in California when he made his ill-fated trip.

According to Jill Tahmooressi, her son was living out of his pickup truck while he sought transitional housing when he left a shopping center parking lot around 11 p.m. and turned down a dark road that led to the border crossing and his current nightmare.

Tahmooressi hopes that the Mexican attorneys she has hired will be able to convince the judge to find it was an accidental entry and drop the charges. A hearing is scheduled for May 28, when members of the arresting Mexican border officials and military will need to present their statements to the judge, who could make a ruling then. Beyond that, Tahmooressi’s legal team cannot say when or if the case might go to trial.

Officials from the U.S. Consulate check in on Tahmooressi, but other than that the U.S. government has been unable to intervene on Tahmooressi’s behalf. Rep. Duncan Hunter, R-Calif., is urging the Obama administration to suspend military aid to Mexico.

“Perhaps Mexico should be reminded of the hundreds of military and law enforcement incursions at the border, where officials and personnel have entered the U.S. without permission and most always carrying weapons,” Hunter said in a statement. "These incidents must be stopped altogether, but Mexico’s actions in Andrew’s case, similar to others, underscore the immediate need for a new form of legal treatment by U.S. officials when incursions occur.”

Jill Tahmooressi warned Americans to give the Mexican border a wide berth, and urged the State Department to better inform travelers with regular public service announcements and better signage along U.S. roads near border entry points.

"People should stay 10 miles from the border because it is so dangerous there," she said. "I am appalled we don't protect our people better on this side of the border so Americans are not subjected to Mexican brutality."