(CNN) -- Answers have been elusive on what may have contributed to the February 15, 2011, ambush that killed a U.S. agent and injured another while on assignment in Mexico.
Was Immigration and Customs Enforcement Agent Jaime Zapata's death due in part to negligence by his supervisors? Do the U.S. gun shops that sold the weapons used in the attack share some responsibility? What about the armored vehicle company whose SUV failed to protect the agents?
Now, the family of the slain agent, in a $75 million lawsuit filed Tuesday, alleges that all these and more were responsible for Zapata's death. The court docket originally listed a $75 billion demand, but was amended on Thursday.
A second immigration agent, Victor Avila, was wounded when he and Zapata were ambushed on a highway in San Luis Potosi. The two were traveling to Mexico City in an armored car with diplomatic plates. They were run off the road and attacked from two vehicles by gunmen who fired indiscriminately, Mexican authorities said.
Avila is also a plaintiff in the lawsuit.
The lawsuit names more than 20 defendants, including the U.S. government, the straw purchasers who acquired the weapons used, and the federal firearms officials who oversaw the flawed "Fast and Furious" operation aimed at tracking the flow of weapons across the U.S.-Mexican border.
Some people, including Sen. John Cornyn of Texas, have asked whether a "gun-walking" program similar to Fast and Furious played a role in Zapata's killing.
The lawsuit alleges that as early as 2010, the Bureau of Alcohol, Tobacco, Firearms and Explosives had identified brothers Otilio and Ranferi Osorio and Kelvin Morrison as straw purchasers for Mexican cartels, but did not act to stop them from purchasing more weapons, a pair of which ultimately found their way to the crime scene in Mexico.
"ATF had an opportunity and probable cause to arrest the Osorio brothers and Morrison but failed to do so," the lawsuit states.
It names the departments of Justice and Homeland Security, as well as the supervisors of Fast and Furious, as the group that developed the strategy of letting guns "walk" that was allegedly also used in Texas.
The government "placed weapons in the hands of a dangerous criminal organization and failed to warn, thereby placing agents Zapata and Avila in harm's way," according to the lawsuit.
Tuesday's filing follows a similar lawsuit filed two months ago by the family of slain Border Patrol Agent Brian Terry. His killing in Arizona, near the Mexican border, brought the strategy of gun walking to light. Terry's family sued federal officials and the owner of a gun shop.
The Zapata suit goes further, blaming the agents' supervisors for sending them on the risky trip to pick up a package.
The U.S. Embassy in Mexico had issued a warning to avoid driving in certain regions, and there were other options for delivering the package, the lawsuit states.
"Despite opposition by Avila and despite having full knowledge of the dangers present with the package pick-up, (the supervisors) instructed Avila and Zapata to proceed with the directive," the court document states.
The company that built the armored Chevrolet Suburban that the agents were in also shares responsibility for "faulty safety features" on the vehicle, the lawsuit alleges.
After Zapata was forced to pull over after being boxed in by his assailants, he placed the vehicle in "park," which automatically unlocked the doors, creating the opportunity the attackers needed to breach the vehicle, it says.
Finally, the lawsuit says the gun sellers should have been aware of false statements made by the straw buyers whose gun purchases ended up in Mexico.
"All of these illegal firearm purchases occurred under extremely suspicious circumstances," according to the lawsuit.
One of the gun sellers named in the suit, JJ's Pawn Shop in Beaumont, Texas, says it did nothing illegal.
The president of the pawn shop, Jim Hedrick, said he was not aware of the lawsuit, but remembers the gun purchase in question. The ATF and FBI both went through the buyer's paperwork and approved the sale, he said.
The Immigration and Customs Enforcement agency continues to mourn Zapata's loss and honors his sacrifice, but it does not comment on pending litigation as a matter of policy, spokeswoman Barbara Gonzalez said.
Responses were not immediately available from the other named defendants.
One of the suspects in Zapata's killing, Julian Zapata Espinoza, also known as "Tweety," was arrested in Mexico and extradited to the United States in late 2011.
He is charged in a four-count indictment with murdering an officer or employee of the United States; one count of attempted murder of an officer of employees of the United States; one count of attempted murder of an internationally protected person; and one count of using, carrying, brandishing and discharging a firearm during and in relation to a crime of violence causing death.
Zapata Espinoza pleaded not guilty and is awaiting trial.