MEXICO CITY -- As a U.S. Marine, Jon Hammar endured nightmarish tension patrolling the war-ravaged streets of Iraq’s Fallujah. When he came home, the brutality of war still pinging around his brain, mental peace proved elusive.

Surfing provided the only respite.

“The only time Hammar is not losing his mind is when he’s on the water,” said a fellow Marine veteran, Ian McDonough.

Hammar and McDonough devised a plan: They’d buy a used motor home, load on the surfboards and drive from the Miami area to Costa Rica to find “someplace to be left alone, someplace far off the grid,” McDonough said.

They made it to only the Mexican border. Hammar is in a Matamoros prison, where he spends much of his time chained to a bed and facing death threats from gangsters. He’s off the grid, for sure, in walking distance of the U.S. border. But it’s more of a black hole than a place to heal a troubled soul.

The reason might seem ludicrous. Hammar took a six-decade-old shotgun into Mexico. The .410 bore Sears & Roebuck shotgun once belonged to his great-grandfather. The firearm had been handed down through the generations, and it had become almost a part of Hammar, suitable for shooting birds and rabbits.

But Mexican prosecutors who looked at the disassembled relic in the 1972 Winnebago motor home dismissed the U.S. registration papers Hammar had filled out. They charged him with a serious crime: possession of a weapon restricted for use to Mexico’s armed forces.

Hammar isn’t the only American accused of questionable gun-related charges at Mexico’s border. Last April, a truck driver who was carrying ammunition through Texas got lost near the border, dipped into Mexico to make a U-turn and was forced to spend more than six months in jail.

It’s been months since Hammar’s Aug. 13 arrest, and his former Marine comrades are livid and dumbfounded, impotent to help.

“It’s heartbreaking. This is a guy who I served with in numerous combat situations, and he was one of the best we had,” said veteran Marine Sgt. James Garcia.

Hammar, 27, joined the Marines and deployed to Afghanistan and Iraq before receiving an honorable discharge in 2007, serving another four years in inactive reserve. In Fallujah, one of the most dangerous cities in Iraq, Hammar’s Marine battalion was hit hard, with 13 killed in action and more than 100 wounded, Garcia said.

“There were days where it was like, dude, I may not make it out of here,” Garcia said. “If it wasn’t the IEDs, it was the car bombs or the suicide bombs.”

In Afghanistan, the Marine unit provided security for President Hamid Karzai, protected election polls and disrupted insurgent cells around Kabul.

Hammar did not have an easy re-entry to civilian life. After recurring bouts of depression, he voluntarily checked into The Pathway Home, a residential treatment center for veterans in California’s Napa Valley, in August 2011 for treatment of post-traumatic stress disorder. He graduated nine months later.

“A big portion of his PTSD is survivor’s guilt. It’s a loss of innocence,” said Olivia Hammar, his mother, a Miami-Dade County magazine publisher. “You’re still trying to process all your friends who didn’t come home.”