(CNN) -- A U.S. Marine Corps reservist held in a Mexican prison for more than three months on a weapons charge could have been released within days of his detention if not for "missed opportunities" by his original legal counsel, his attorney told CNN.
In an exclusive phone interview, Fernando Benitez -- the latest attorney for jailed Sgt. Andrew Tahmooressi -- said his client's case was allowed to "simmer" by his original attorneys. They have since been fired.
"For anybody being involved in a criminal case in Mexico, the first 96 hours ... those first 96 hours are crucial," Benitez said. "A lot can be done, and releases can and are obtained regularly, but you have to aggressively address a defensive strategy."
Now, Benitez says, his client is at the mercy of the Mexican judicial system, and there is no timetable for his release.
Tahmooressi has maintained that he took a wrong turn on the California side of the border into Tijuana, Mexico, the night of March 31. His mother told CNN in May that Tahmooressi, who served in Afghanistan, had moved to the San Ysidro, California, area to get treatment for post-traumatic stress disorder.
He was detained by Mexican border officials for possessing a .45-caliber pistol, a 12-gauge pump shotgun and an AR-15 rifle.
For weeks, Tahmooressi sat in a Tijuana prison while his defense attorneys failed to submit evidence to the court in the initial stages of his defense.
Tahmooressi and his attorneys alleged that he was beaten and tortured by guards and prisoners at the prison. Mexican officials have denied that claim.
He has since been transferred to El Hongo Penitentiary in Tecate, where he told CNN earlier this month that he is being treated well.
Benitez has submitted evidence and will continue to do so between now and August 4, Tahmooressi's next court date. His combined legal bills have already surpassed more than $20,000, according to the family.
"In Andrew's case, several windows of opportunity were missed," Benitez said. "It's frustrating to us. I would have loved to have taken advantage of those opportunities. But now we need to work with what we have and that's exactly what we're going to do."
Germane to Tahmooressi's defense is that Tahmooressi's rights were violated under the Mexican constitution, Benitez said.
"I believe he was denied several basic human rights, which, it's my contention, should result in reparation from the court" in the form of declaring a mistrial or tossing out the testimony "of those officers who are singling him out as the responsible party in this case."
A 20-year criminal defense veteran, Benitez said he has a strong track record of acquittals or cases being thrown out in federal court.
In 2011, he defended former Tijuana Mayor Hank Rhon, whose house was raided by Mexican soldiers who discovered an arsenal of illegal guns and ammunition. Benitez successfully argued that the raid was performed without a warrant. All charges were dropped.
The same should be expected for Tahmooressi's case, Benitez told CNN.
"Andrew's case, believe it or not, is technically not that complex," Benitez said. "It's a mistake many people have made. I've driven my car up to San Diego and back to Tijuana and I can tell you I've made the same mistake he made. And I'm a resident. The evidence we've been uncovering all supports his story."