It might be the first order U.S. Marine Sgt. Andrew Tahmooressi doesn't obey.

The mother of the reservist, who is being held in a Mexican jail after mistakenly crossing the border with registered guns, told FoxNews.com her son just got an "Order to Muster" letter from the Marines telling Tahmooressi to report to the 4th Civil Affairs Group in Hialeah, Fla., on June 14. Although a Marine spokeswoman later told FoxNews.com that the corps is aware of Tahmooressi's current circumstances and does not expect him to report, failure typically can result in "other than honorable discharge" and affect Veterans Administration benefits.

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"You are among the elite citizens of our nation who, if needed, are ready to answer the call to defend our freedom," reads the May 9 order written by Sgt. Maj. M.E. Sprague, which also reminds the recipient that, "Once a Marine, always a Marine."

Jill Tahmooressi said the letter was the latest painful reminder of her son's plight despite service to his country that includes two tours of duty in Afghanistan resulting in post-traumatic stress disorder. Tahmooressi was arrested March 31, after accidentally driving into Mexico with three legally purchased weapons in his truck.

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A Marine Corps spokesman told FoxNews.com reservists on Individual Ready Reserve, as Tahmooressi is, are contractually required to attend the one-day session to have their physical readiness evaluated should they be recalled to active duty. The letter does not represent a call-up to active duty.

Jill Tahmooressi, who has been able to speak to her son by telephone, said she did not know if the Marines sent the letter mistakenly, or did not know of his situation.

"I haven't called the Marines about this," she said.

Maj. Tamarra Jones, a Marine Force Reserves spokeswoman, said the corps is fully aware of Tahmooressi's situation and has been following it closely.

"This is an exceptional circumstance and Sgt. Tahmooressi won't be penalized for not attending this meeting," Jones told FoxNews.com.

Tahmooressi, who joined the military in 2008, was a .50 caliber gunner in the top position of a Humvee, according to his mother. His bravery in combat earned him a battlefield promotion to sergeant.

The fearful mom hopes her son's training and bravery will sustain him through his current ordeal. Tahmooressi attempted an escape from a notorious state-run jail in Tijuana shortly after his arrest. He has since been transferred to a federal penitentiary in Tecate, Mexico.

His first court hearing is scheduled for May 28, but will likely not result in any decision. The arresting Mexican border officials are slated to make a statement to the judge, who holds Tahmooressi's fate in his hands alone. There are no jury trials in Mexico.

Meanwhile, a petition drive has been launched to spur President Obama's intervention in the case. In order for the White House to automatically look into the case, 100,000 signatures must be gathered by May 31. As of Tuesday, there are about 30,000 signatures attached to the petition.