A former U.S. Marine held for more than three months in a Mexican jail on weapons charges appeared in court in the border city of Tijuana on Wednesday, his lawyers said.

Andrew Tahmooressi, 25, a veteran of the war in Afghanistan, was arrested on March 31 by Mexican customs agents who found guns in his pick-up truck at the San Ysidro border crossing between San Diego and Tijuana.

His supporters, including U.S. Representative Duncan Hunter, a Republican from California, say Tahmooressi drove there with three registered firearms after making a wrong turn and never meant to enter Mexico.

Fernando Benitez, a lawyer for the former Marine, said defense lawyers planned to question on Wednesday two witnesses who work for the customs authority.

"It's important because they had him for seven and a half hours in customs," Benitez told Reuters in Mexico City. "After that ... we will have the elements to be able to begin the defense."

A supporter in contact with Tahmooressi's mother Jill said he fired a previous lawyer who had advised him to lie and say he had never before used that border crossing and was new to the San Diego area, when both were untrue.

Supporters say Tahmooressi, who left the Marines in 2012, called 911 after realizing he had entered Mexico by accident with the three guns and 400 rounds of ammunition in his truck.

In May, the Mexican customs administrator in Tijuana told local media there the former Marine did not cross into Mexico by accident, and that he did not say when he arrived that he had weapons in his possession.

In May, Hunter asked U.S. Secretary of Defense Chuck Hagel to consider suspending military training and equipment assistance to Mexico until the case is resolved.

(Reporting by Liz Diaz; Writing by Daniel Wallis; Editing by Eric Walsh)