Updated

The third attorney to take up the case of Andrew Tahmooressi, the U.S. Marine sergeant held for three months in a Mexican prison after mistakenly crossing the border with registered guns, said only the legal process - not diplomacy - can get his client back to America.

Defense attorney Fernando Benitez, who will represent the Afghan war veteran in a Tijuana court for the first time on July 9, said demands, threats and diplomacy from American politicians will have no effect on Tahmooressi's release.

"This is a federal court and as in any democratic nation you will not find an executive calling up a judge and ordering the release of a suspect," Benitez said. "It makes no difference if it is a mayor, governor, or the president, there's not a phone call in the world that will change this."

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Tahmooressi's supporters have grown impatient as the case has slowly wound through the Mexican judicial process, delayed by the firing of his first two attorneys, who sources said did little but send the Tahmooressi family bills. Several backers have called for President Obama to personally appeal to Mexican President Enrique Pena Nieto, but Benitez said that would likely be a waste of time.

Still, lawmakers led by Rep. Duncan Hunter, R-Calif., have been calling for his freedom since Tahmooressi was arrested at the Tijuana Port of Entry March 31. He alleges he accidentally drove into Mexico confused by poorly placed and lit freeway signs. He was charged with possession of weapons and ammunition used by the Mexican military. Both, according to Benitez, are very serious charges and could result in some 10 years in prison if he is convicted.

Benitez said the only real diplomatic mechanism to secure Tahmooressi's release would be a prisoner exchange, but he said such a strategy makes no sense given  Tahmooressi's innocence, which he vowed to demonstrate in court.

"There are agreements in place between the U.S. and Mexico for such things and have been done in the past," Benitez said. "But why would you want to take back convicted criminals from another country?"

Benitez defended the Mexican legal system, and urged patience.

"We've always had a presumption of innocence," Benitez said. "The whirlwind of media attention is well and fine but no evidence has even been presented."

Benitez said he is comfortable with the presiding judge, Victor Octavio Luna Escobedo, saying he is young and a "stickler for formality."

When Tahmooressi's first attorney, Alejandro Osuna, was fired the night before his May 28 hearing, the judge declined to let a public defender assume the case in order to avoid loss of continuity of counsel.

"He is being very respectful to due process," Benitez said.

As for who the prosecutor is, that Benitez said is essentially a dice roll. In the Mexican legal system, unlike in the U.S., a case really doesn't have a lead prosecutor, and each hearing could have a different lawyer representing the prosecutor's office preside. At this time, he doesn't even know who he would face during the actual trial but could check submitted court documents to find out.

While preferring not to disclose his defense strategy, Benitez said he will raise human rights violation issues.

"He was held at the port of entry for seven hours with no translator or knowledge of what was going on," Benitez said of Tahmooressi. "There were so many issues to be raised before he even saw the prosecutor."

Benitez declined to critique the work of two lawyers who preceded him, but said he would have worked to have the case dismissed back in April. For now, he said, Tahmooressi is likely going to have to endure the legal process that could take months before a trial is even heard.