A lawyer for a U.S. Marine being held in a Mexican prison on gun charges told Fox News’ Greta Van Susteren Tuesday night that he feels “a little bit stonewalled” by Mexican authorities as he works to fight for his client’s freedom.

Sgt. Andrew Tahmooressi was arrested by the Mexican military on March 31 after border inspectors found three legally purchased and registered firearms 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.

His attorney, Alejandro Osuna, said on “On the Record with Greta Van Susteren” that Tahmooressi’s evidentiary hearing on May 28 will be very important to the case, because he will be able to “yank out the truth about what really happened” when the Marine crossed the border.

Osuna said proving the truth is so important because he recently read a claim that Tahmooressi  attempted to drive through customs without stopping, which he said is “completely false.” Osuna said he recently subpoenaed Mexican customs officials for surveillance video of the alleged incident, but has not received it.

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

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