On the Record has followed every twist and turn in the case of a U.S. Marine who has been held for months in a Mexican prison. Now tonight at 7p ET, Greta Van Susteren brings you an exclusive phone interview with Sgt. Andrew Tahmooressi.

He was arrested March 31 after he crossed into Mexico with three guns in his truck.

In the clip above, the 25-year-old describes what happened after he mistakenly crossed the border at the San Ysidro checkpoint. He had called 911 moments earlier, saying that he had made a wrong turn and feared that he could get into trouble because of the registered guns.

"[A Mexican border guard] got on the walkie-talkie and was communicating what was going on," Tahmooressi said. "I think what he said was, 'Hey we've got a guy down here with three guns.'" 

Then, he recalled that a Mexican military officer showed up at the scene like a "man on a mission."

"He just took control from there," Tahmooressi said of the officer. "He didn't seem to care at all about anything that I had to say or my story. It was like a math equation in his head. Three guns, man, equals prison."

Tahmooressi fired his attorney Alejandro Osuna, who had earlier advised Tahmooressi to claim that he had never been to Tijuana. Mexican officials said at a court hearing Wednesday that Tahmooresi lied to them and had traveled to Mexico several times before his arrest, including earlier on the day of his arrest.

Watch the full interview with Sgt. Tahmooressi on On The Record, tonight at 7p/2a ET.