Pallbearers carry the casket of ICE agent Jaime Jorge Zapata, who was shot and killed in Mexico as he traveled with another agent last year, to the Rose Lawn Garden Memorial cemetery. (DELCIA LOPEZ/AP)

When U.S. special agent Jaime Zapata was shot dead one year ago on a notorious stretch of highway in central Mexico, he was driving a $160,000 armored Chevy Suburban, built to exacting government standards, designed to defeat high-velocity gunfire, fragmentation grenades and land mines.

But the vehicle had a basic, fatal flaw.

Forced off the road in a well-coordinated ambush, surrounded by drug cartel gunmen brandishing AK-47s, Zapata and his partner, Victor Avila, rolled to a stop. Zapata put the vehicle in park.

The door locks popped open.

That terrifying sound — a quiet click — set into motion events that remain under investigation. When Zapata needed it most, the Suburban’s elaborate armoring was rendered worthless by a consumer-friendly automatic setting useful for family vacations and hurried commuters but not for U.S. agents driving through a red zone in Mexico.

The Feb. 15, 2011, killing of Immigration and Customs Enforcement Special Agent Zapata — the first U.S. officer to die in the line of duty here since 1985 — is described by American officials today as a watershed in the U.S.-backed drug war and proof that the United States is paying in blood as well as treasure.

His death has put new focus on the expanding role that a growing number of U.S. law enforcement officials now play in Mexico, where they operate in an increasingly complex, dangerous environment — yet are prohibited by Mexican law from carrying firearms to protect themselves.

Piecemeal accounts of the attack have trickled out over the past year, but the U.S. government has not made public key details about the circumstances that led to Zapata’s death. It is still unclear why the agents were sent down a stretch of highway where gangs were known to roam, because they could have flown or traveled with an armed escort of Mexican military or police, a routine matter.

But in interviews with U.S. lawmakers, senior law enforcement officials and security experts, it appears the officers were sent on a dangerous mission in a vehicle whose basic protections were disastrously flawed.

Had the door locks not opened at that moment, “Zapata would probably still be alive today,” said Rep. Michael McCaul (R-Tex.), a former U.S. prosecutor who is pressing U.S. agencies to clarify what happened in the attack, in which Avila was wounded.

There are hundreds, if not thousands, of other armored U.S. government vehicles all over the world that may also have this vulnerability, potentially leaving American diplomats and officers at grave risk.

The Bureau of Diplomatic Security, which is responsible for outfitting the secure vehicles used by American embassies abroad, declined to comment on the status of their locking mechanisms.

But when McCaul visited Afghanistan last month and rode in an armored State Department Suburban, he said he paid close attention to what happened when the vehicle went into park.

The doors unlocked.

The ambush

Zapata, 32, was only nine days into a temporary assignment in Mexico when he was killed, according to his family. He told his worried parents back home in Brownsville, Tex., that he would be stationed at the U.S. Embassy, pushing paper in the relative safety of Mexico City.

Instead he was at the wheel of a big Suburban with tinted windows, barreling through the scrublands along Highway 57 to courier what U.S. officials describe as a piece of “sensitive electronic equipment” from Monterrey to Mexico City.

It is a busy four-lane highway. But it is also at the heart of what veteran U.S. agents consider a perilous region, ruled by the carjackers, kidnappers, dope smugglers and hit squads who work for the most violent criminal organization in Mexico, Los Zetas.

The two agents were confident enough to stop along the road at a Subway sandwich shop. It was here that U.S. officials say lookouts for the Zetas likely spotted what appeared to them to be two young, well-built Mexicans, speaking fluent Spanish and driving the exact kind of SUV that attracts attention here, a vehicle favored by three types of drivers: government officials, wealthy elites and powerful gangsters.

A couple dozen miles south of the state capital of San Luis Potosi, the agents realized they were being followed. Driving aggressively, in front and back, two SUVs with eight assailants quickly boxed in the agents.

Mexican investigators are operating under the theory that the Zetas wanted to carjack the Suburban or believed it was being driven by rivals from the Gulf Cartel — though it is still possible they were targeted because they were American agents.

While reluctant to criticize fellow officers, senior law enforcement agents stressed that in an ambush you never stop. “You get off the X,” they explained, meaning out of the kill zone. That is what an armored vehicle is designed to do. With re-enforced “ram bumpers,” heavy-duty shocks and “run-flat” tires, the vehicles are not only secure shells, but weapons.

With the Suburban on the side of the road and unlocked, the Zeta gunmen wrestled Avila’s passenger door open. As the agents screamed that they were Americans and diplomats, Avila or his attackers hit the button that rolled the bulletproof window down. The gap was just four inches but enough for one gunman to get the barrel of his AK-47 into the Suburban and rake gunfire across their laps.

“Avila described them as in a frenzy, as pure evil,” said McCaul, the Texas congressman. Avila was hit twice, in the thigh and ankle, and grazed by two other shots, but he managed to close the window and secure the door. Zapata took six bullets. One severed his femoral artery.

As Zapata bled to death in the driver’s seat and Avila placed a frantic call for help to the U.S. Embassy in Mexico City, the gunmen pumped 90 more rounds into the Suburban. None penetrated its armor.

The Suburban driven by Zapata was outfitted by Arlington-based BAE Systems, a U.S. subsidiary of the British defense contractor, maker of tanks, submarines and jet fighters.

Disabling the unlocking mechanism on a Suburban is a relatively simple process, armoring specialists say. The setting can be adjusted on the vehicle computer by the driver or permanently altered in favor of a manual system.

U.S. officials say all of the vehicles in Mexico were reprogrammed to address the flaw after Zapata’s death. But armoring contractors point out that if the computer on the Suburban is reset — by a power failure or a battery replacement — it will revert to the default setting, leaving the vehicle vulnerable again.

BAE Systems spokesman Brian Roehrkasse said in a statement, “The government customer was aware of the importance of disengaging the auto unlock feature.”

A gun from Texas

U.S. investigators recovered one of the military-style semi-automatic weapons used in the attack that killed Zapata.

The gun came from Texas.

Ballistic testing of spent shell casings and the raising of an obliterated serial number revealed the weapon was a popular Romanian-made AK-47 knockoff purchased at J&J’s Pawn Shop in Beaumont, smuggled south to the Zetas by a methamphetamine trafficker named Manuel Gomez Barba, a U.S. citizen.

A congressional inquiry is underway to determine whether the Bureau of Alcohol, Tobacco, Firearms and Explosives allowed the weapon to “walk” across the border, just as the agency lost track of thousands of other firearms that were smuggled to Mexico through the disgraced operation known as Fast and Furious.

U.S. diplomats said American agents in Mexico follow the law, which forbids them from carrying weapons. But several government sources with knowledge of the ongoing investigation say Zapata and Avila were armed on the day of the ambush, though they were outgunned.

The sources requested anonymity because the issue is politically explosive in Mexico, where foreigners are prohibited from carrying firearms.

“Our guys have the ability to defend themselves,” one U.S. official said.

Capitol Hill lawmakers say they will press the State Department to force the issue with their Mexican counterparts. “We know they have their own laws,” said Rep. Henry Cuellar (D-Tex.). “But on the practical side, the Mexicans need to understand that we can’t let this happen again. We can’t leave our guys helpless.”

A frustrated family

In the weeks after the Zapata killing, thousands of U.S. agents and local police arrested and interrogated hundreds of suspected associates of Mexican drug cartels across the United States.

In Mexico, acting on U.S. intelligence from confidential informants and wiretaps, the Mexican military rounded up murder suspects and their bosses.

In December, Mexican authorities extradited an alleged low-ranking soldier in the Zeta cartel to the United States to stand trial for Zapata’s death in Washington. Julian Zapata (no relation), known by his moniker “Tweety Bird,” has pleaded not guilty.

Jaime Zapata grew up in a family with a tradition of serving in U.S. law enforcement. Two of his brothers are ICE agents. But the Zapata family has grown “extremely frustrated,” said Trey Martinez, their attorney.

“They want to know why he was put on that road. What equipment was so important that he had to drive up and bring it down? If it was so dangerous, why didn’t they have an escort? Or fly?"

And why, he said, hadn’t anyone checked the settings on the Suburban’s door locks?

“The government has been good about providing the family with emotional support,” Martinez said. “But the same people that are helping them aren’t giving them the answers they’re looking for.”