Updated

Whether U.S. Marine Sgt. Andrew Tahmooressi spends the next two decades in a Mexican prison or is freed tomorrow for mistakenly crossing the border with loaded guns all rests with an unpredictable legal process that bears little resemblance to the American court system.

Tahmooressi, 25, of Weston, Fla., has been held in two Mexican prisons since being arrested while accidentally crossing into Mexico on March 31 with three weapons in his truck. According to Tahmooressi’s defense attorney Alejandro Osuna, of Tijuana, Mexico, his client is facing federal charges for carrying a weapon intended for exclusive use by the military, carrying a weapon not registered in Mexico and possession of ammunition.

“These are very serious charges in Mexico,” Osuna said.

Tahmooressi alleges that he made a wrong turn while leaving a San Ysidro, Calif., parking lot late on March 31 and was unable to avoid entering Mexico. Once across the border, he tried to turn around immediately. Osuna said his client fully disclosed to the customs agents he had weapons in his truck and that he did not intend to enter Mexico.

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“Andrew said that one of the customs officials offered to escort him back across the border,” Osuna said. “We need to find out what happened at that point."

Osuna said there are few similarities between the Mexican and U.S. legal systems which adds to misperceptions and glacial speed in getting evidence entered and argued. The most glaring difference is that there's no jury — a judge will decide Tahmoressi's fate alone, unless a top government official intervenes.

“Unlike the U.S. system, where you will have a two or three day trial, in Mexico evidence and hearings are presented piecemeal,” Osuna said.

Tahmooressi’s first court session is slated for May 28, where he will face the two customs agents and two soldiers involved in his arrest and be able to make a statement.

“We want him to explain everything that happened the night of his arrest,” Osuna said.

In Tahmooressi’s favor is a 911 call he made explaining his plight and failed attempt to get back into the U.S. Osuna said he will also subpoena the video of the customs station when his client was arrested.

But those pieces of evidence, as Osuna predicted, won’t be disclosed until later dates. A subsequent hearing to listen to the 911 tape is scheduled for June 4. On June 5, a court officer will accompany the lawyers to the actual border crossing.

David Shirk, a political science professor at the University of San Diego, where he heads the Justice in Mexico Project, said the border ports of entry are complicated but that he understands Mexico’s response.

“There are giant signs saying you are about to enter Mexico and there is a U-turn option about 300 yards before the entrance to Mexico,” Shirk said. “I do believe that this was a mistake, but it is understandable that Mexico would believe that adequate warning was given.”

Shirk said the case touches on not only differing legal systems, but issues of sovereignty between the U.S. and Mexico.

“You might think about this case in the following way: what if a US citizen in possession of legally purchased medical marijuana were to accidentally enter Mexico? That person would be in violation of Mexico's prohibition on marijuana, and would be expected to face justice,” he said. “Or what if a Mexican citizen in possession of a small quantity of heroin, which would be a acceptable under Mexico's minor possession laws, enter the United States port of entry accidentally? Would we not expect that person to be held to account? Or would we say, 'Whoops, they made a mistake, let's send them back to Mexico?'”

Both Osuna and Shirk agree that Tahmooressi faces an arduous legal ordeal. The judge can dismiss the case at any point, but the decision is his to make.

“The judge won’t dismiss the case at this time,” Osuna said.

Osuna added that the mounting political and diplomatic pressure the U.S. is generating for the release of Tahmooressi, it is unlikely a release will be imminent.

“The judge who is hearing the case in Tijuana is oblivious to the American media,” Osuna said.

Rep. Duncan Hunter, R-Calif., requested a pardon from the governor of Baja State where Tijuana is located, but because Tahmooressi is facing federal charges, there is little a state official could do.

Not even Mexican President Enrique Pena Nieto has the option to pardon prisoners, according to Shirk, who added that he's not certain if the new federal rules for criminal procedure are yet in effect to allow the attorney general to opt out of prosecuting a case.

“The only way he would get out at this point if from an order from Mexico’s attorney general,” Osuna said.

Even that is a long shot, although the U.S. State Department is said to be pressing behind the scenes.

“It is not clear to me that there is some kind of get out of jail free card,” Shirk said.  “Assuming that this does go to trial, it could be months or years before all of the evidence is brought to bear that is needed for the case. In the meantime, he would likely need to remain in pretrial detention. “

In December 2012, a Mexican federal judge ordered the immediate release of marine Jon Hammar, 27, who had crossed into Mexico the previous August with an antique rifle in his vehicle en route to a surfing trip. Like Tahmooressi, Hammar also suffered from severe combat-related PTSD.

The Mexican court ruled that Hammar had no criminal intent to bring the antique 60-year-old rifle across the border and had his constitutional rights violated.

Hammar's relatives were subjected to extortion calls from jailed cartel members and the pain of seeing their son chained to a bed in the administrative offices of the Matamoros State prison.

Tahmooressi will remain behind bars in a state penitentiary in Tecate, Mexico, some 40 miles east of Tijuana.