Operation Fast and Furious was a plan to infiltrate criminal gangs in Arizona and allow them to illegally buy guns so agents could see the routes traffickers used to smuggle weapons into Mexico for use by violent drug cartels.

But somehow the operation was bungled and U.S. firearms agents allegedly lost track of nearly 2,000 weapons, some of which have been linked to the killing of a U.S. border patrol officer.

On Tuesday officials testified before a congressional hearing in Washington, D.C., to explain how the plan went so dangerously wrong.

Police crackdowns have failed to control much of the trade in drugs and guns running north and south across the Mexico-U.S. border.

So, two years ago, agents with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives in Phoenix developed a strategy.

Instead of busting low-level gun buyers, they would instead try to track the guns, videotape and tail the suspects, all in hopes it would lead to the bosses.

But then Brian Terry, a border patrol officer, was shot dead last December, and guns found at the scene were among those the ATF was supposed to be keeping an eye on.

The patrolman's family is demanding answers.

"Brian was about making a difference in justice and I just feel that this country owes it to him," said Terry's sister, Michelle Terry Balough.

Details emerged at the hearing Tuesday about how agents in Arizona allegedly lost track of nearly 2,000 handguns and high-powered assault weapons.

Hundreds of weapons remain unaccounted for, but are showing up on the killing fields of northern Mexico.

Around-the clock surveillance failed as the ATF ran out of money or its agents were plain outwitted.

William Newell, the former special agent in charge, said he never intended for the guns to make their way across the border.

"The purpose of this investigation was to identify, and disrupt and dismantle, an entire firearms trafficking organization that was linked to a Mexican drug cartel," he told the congressional committee.

Committee members seemed frustrated by his answers.

"It continued on and on, and consequently there were thousands of weapons that ended up in Mexico, killing people. Killing people. That's the reason we are here today," said Republican congressman Jason Chaffetz.

Newell said to the best of his knowledge, "we did not let guns walk."

Operation kept secret

The operation was kept secret from Mexican officials, nor did the ATF tell its own agents stationed in Mexico.

Some of them are outraged, leading them to testify against their Phoenix colleagues.

"To put it bluntly, it is inconceivable in my mind, or the mind of any competent ATF agent, to allow firearms to cross an international border, knowing that they are destined for the worst of the worst criminals in the Western Hemisphere," said Darren Gil, who used to run the Mexico ATF operation.

Newell said the ATF had to try something, instead of year after year arresting only the so-called straw buyers willing to buy AK-47's on behalf of drug kingpins.

"These Mexican drug cartels are going to get their firearms. They're going to get them. Straw purchasers are the lowest rung on the ladder. They're like a street corner drug dealer. If you just focus your investigations on straw purchasers, you're not having a lasting impact," Newell said.

The program is now shut down, after 19 people were arrested — only one of them for gun trafficking.