The father of a U.S. Marine jailed in Mexico after being caught with his grandfather's antique shotgun heard the fear in his son's voice and felt helpless.

The phone call came at midnight from Mexico's notorious CEDES prison, where Jon Hammar Jr. has been held since August. The caller demanded $1,800, then put Hammar on to drive the point home.

"They're serious, Dad," Jon Hammar Sr. heard his son say. "I'll pay you back; they are going to kill me."

"They're serious, Dad. I'll pay you back; they are going to kill me." — Jon Hammar, Jr., speaking by phone from a Mexican jail

Hammar, who faced down Iraqi insurgents in the final push on Fallujah in 2004, has been in dangerous situations before. But his treatment in the infamous prison, where Mexico's murderous Los Zetas and Gulf drug cartels hold sway, has his family fearing for the 27-year-old's life -- and begging the Obama administration for help. Hammar was arrested in the Mexican border city of Matamoros on Aug. 13, after declaring to a Mexican customs agent that he possessed an antique shotgun he was carrying through the country on his way to Costa Rica, where he and a pal planned to surf and forget the horrors of war that plagued Hammar long after his honorable discharge in 2007.

Even though a U.S. border agent in Brownsville, Texas, had assured Hammar the gun was legal as long as he declared it to Mexican authorities, he was nabbed just across the border, and charged with an aggravated felony punishable up to 15 years in prison. While in prison, Hammar has been repeatedly threatened and, according to reports, left chained for days to a steel bed. But it was the call, just two days after their son's arrest, that continues to haunt Hammar's parents. They believe their son's service to his country -- memorialized forever by a "USMC" tattoo on his arm, made him a target behind bars.

"The reason why he got processed so fast was because he has a USMC tattoo," Jon Hammar Sr., 48, told FoxNews.com. "You can't mistake who these guys are."

It was Hammar's mother, Olivia, who took the unnerving call at their Palmetto Bay, Fla., home. As her face turned ashen at the caller's demand, Jon Hammar Sr. grabbed the phone and heard a voice say, "This prison is our house!"

It wasn't an idle boast. The prison was the scene of the escape of 151 inmates in December 2010 and 59 in July 2011, and dozens of guards were later charged with helping with the breakout. And the prison has an unparalleled reputation for violence: In 2005, two American brothers jailed on homicide charges were found stabbed to death in their cells. The inmate ranks are swollen with members of the Mexican mafia and various cartels, shootouts and escapes are common and guards have confiscated guns and even an AK-47s from cells over the years.

Hammar could tell his son was under duress. He was fully prepared to pay the ransom, but the caller said he would call back in the morning with a Western Union account number. Hammar found that to be strange.

"You're about to kill my son and you don't even have an account number and you'll call me back?" Hammar said.

Hammar also wondered how the caller got his home number and was able to place the nighttime call. He got hold of a U.S. Consulate official who promised to convey the threat to high-ranking Mexican military officials in the region. No call came from the prison in the morning.

A shaken Hammar knew that he had to get to his son as soon as possible. After negotiating the procedural maze of obtaining consulate approval to go to the prison, Hammar and his son's attorney, Eddie Varon-Levy, made their way to Tamaulipas, the northeastern state where the prison is located just 15 miles from Brownsville, Texas. They were surprised to find that when they arrived, consular officials could not obtain clearance to accompany them behind the walls of the lockup. The men went inside alone.

On the surface, CEDES officials appeared to go out of their way to make "everything look good," Hammar recalled. His son, who was not expecting the visit, was shocked and worried when he saw them.

"He wasn't concerned about his safety, but ours," Hammar said. "He was more angry that we put ourselves at risk for coming to the prison."

His son looked worn out and thin, Hammar recalled. What concerned him most was how "odd" the side of his mouth looked and his son's reluctance to show his father his body.

"He was wearing his own clothes and when I went to lift his shirt up he knocked my hand away," Hammar said. "He wouldn't take his shirt off because he was concerned about repercussions."

There are numerous reports and allegations of inmate abuse by other inmates and prison officials in Mexico, some of which have resulted in death.

Ricardo Alday, a spokesman for the Mexican Consulate in Washington, D.C., told FoxNews.com Hammar's safety is guaranteed by the Mexican government.

"Mr. Hammar is currently detained in Tamaulipas and, as any other detainee facing criminal charges, he has the right to defense counsel and a fair trial," Alday said. "In addition, his life and integrity are protected by national and international laws."

Alday said Mexican authorities have ensured Hammar's right to help from U.S. diplomatic officials, and said he has been in contact with U.S. Consular officers in Mexico who have regularly visited him.

A spokesman for the State Department said officials have visited Hammar three times, spoken with him by phone and contacted prison officials to stop them from chaining him to the bed.

"The safety and well-being of U.S. citizens is something we take very seriously," said Peter Velasco.

But his father's confidence in the U.S. State Department has waned as his son has languished in prison.

"We're grateful that they saved his life and are being another set of eyes, but they haven't been much help getting him released," Hammar said.

Their son's PTSD also concerns the Hammars. After repeat combat tours in Iraq and Afghanistan, the wars took an emotional toll on Hammar.

His father said that Jon had received treatment through the Veterans Administration, but had a "very bad" reaction to the medication he was given and was reliant on therapy to help him cope.

"He has not been given any care in prison and, so far, we haven't seen any flare-ups," Hammar said. "We are still concerned."

Joseph J. Kolb is a regular contributor to FoxNews.com