U.S. Marine Sgt. Andrew Tahmooressi has been in Mexican prison since March 31, and the journey to set him free has been slowed by obstacles.

Tahmooressi was arrested by Mexican authorities on March 31 and sent to Tijuana's La Mesa State Prison after crossing the border in a truck filled with three registered firearms. The sergeant claims he crossed the border by mistake as he made a wrong turn due to a lack of clear labeling.

Tahmooressi's evidentiary hearing is on May 28, according to his lawyer, Alejandro Osuna. Osuna told Fox News' "On the Record with Greta Van Susteren" that this hearing will be the moment when he can finally "yank out the truth about what really happened." According to the lawyer, claims that the sergeant went through customs and didn't stop are "completely false." To prove this, Osuna said he asked customs officials in Mexico for surveillance footage from March 31, but they have not complied.

"In fact, we have been getting a little bit stonewalled by the customs administrators," he said.

Osuna added that customs authorities wouldn't bring in a forensics team or take photos of the scene until a judge gave the order.

Meanwhile, the sergeant's mother, Jill Tahmooressi, received a painful reminder of her son's situation this week. He reportedly got an "Order to Muster" letter from the Marines ordering him to go to the 4th Civil Affairs Group on June 14, something he obviously cannot do given his imprisonment.

"I haven't called the Marines about this," the mother told Fox News.

A Marine Force Reserves spokeswoman said that despite sending the letter, the marines are aware of Tahmooressi's situation.

"This is an exceptional circumstance, and Sgt. Tahmooressi won't be penalized for not attending this meeting," the spokeswoman, Maj. Tamarra Jones, said.

A petition calling for intervention from President Barack Obama is currently hoping to get 100,000 signatures, the number required for the White House to respond. As of today, it has over 45,000 signatures.

Tahmooressi was transferred to El Hongo Federal Penitentiary after an incident where he tried to escape by climbing the prison wall and another when he hurt himself with a light bulb in his special cell.

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Follow Scharon Harding on Twitter: @ScharHar.

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