MEXICO CITY - 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, 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 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, when 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-Texas, who is pressing U.S. agencies to clarify what happened.

There are potentially thousands of U.S. government vehicles all over the world that may also have this vulnerability, potentially leaving American diplomats and officers at grave risk. 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.