For months, a mystery has engulfed the U.S. southern border and Mexico—what suddenly caused federal agents to abandon years of practice and knowingly let suspected straw buyers for Mexican drug gangs walk off with semiautomatic weapons from American gun shops?

The answer leads to previously undisclosed instructions given by higher-ups inside the Obama Justice Department, which originally denied any role in the burgeoning controversy, The Daily Beast has learned.

The U.S. government’s Fast and Furious operation—which was designed to build criminal cases against Mexican gun traffickers but went awry—has stirred debate in two countries after revelations that frontline agents vehemently objected to their supervisors’ order to knowingly let guns be trafficked by suspected straw buyers and that hundreds of the guns they were ordered to “let walk” ended up being used in subsequent crimes, including murders.

The issue prompted two investigations in Congress, outrage among Mexican legislators, anger among residents in crime-beleaguered communities on the border—and even forced President Obama to suggest “serious mistakes” may have been made.

Before the gun-running sting began in Phoenix in late 2009, Bureau of Alcohol, Tobacco, Firearms, and Explosives agents were trained to build cases quickly against people who were acting as straw buyers, purchasing cheap legal guns at U.S. gun shops and then transferring them to others who would traffic them to Mexican gangs. Agents have told Congress they wanted to interdict weapons quickly when they fell into the hands of suspected straw buyers and despised the idea of “letting guns walk” outside their control.

But ATF supervisors and local federal prosecutors in the Fast and Furious operation approved a different approach in late 2009, specifically instructing agents along the Arizona border not to interdict the weapons and instead to let the straw buyers move the guns into the system in hopes they would show up in crimes on both sides of the border and help federal prosecutors build bigger cases against the Mexican drug gangs.

The operation went on for 15 months, and ATF officials now concede they let more than 1,700 weapons fall into the hands of the straw buyers, with nearly 800 showing up in criminal activity on both sides of the border. Two of those guns were found at the scene where a U.S. border agent was murdered and more than 190 turned up in Mexican crimes as well.

To date, blame has rested mostly with the Bureau of Alcohol, Tobacco, Firearms, and Explosives supervisors who approved the strategy in Arizona and Washington. The Justice Department also has directed that the tactic of “letting guns walk”—meaning allowing straw buyers to move guns with the government’s knowledge—should no longer be used.

But on Wednesday, The Daily Beast has learned, congressional investigators will disclose that just weeks before ATF supervisors approved the Fast and Furious operation and its controversial tactics, senior Justice Department officials sent a memo to prosecutors and agents on the front lines of the border wars urging that they go beyond their traditional tactics of interdicting guns being purchased by straw buyers and try to make cases against the drug gangs themselves.

“Given the national scope of this issue, merely seizing firearms through interdiction will not stop firearms trafficking to Mexico. We must identify, investigate and eliminate the sources of illegally trafficked firearms and the networks that transport them,” the office of then Deputy Attorney General David Ogden wrote in an October 2009 memo marked law enforcement sensitive.

The memo was prepared in connection with a previously unknown high-level Justice Department meeting in which representatives of key law enforcement agencies on the front lines of the border wars were summoned to discuss a new approach to combating border violence, according to government officials familiar with the document.

Within days, the memo from Ogden’s office was being distributed inside the ATF office in Phoenix, where supervisors quickly launched Operation Fast and Furious.

Government officials confirmed the memo’s contents and the discussions about broadening the border strategy beyond interdiction but insisted the memo provided no specific guidance authorizing the specific tactics of letting the guns “walk.” Attorney General Eric Holder has said he did not know about those tactics at the time and has asked the department’s internal watchdog, the inspector general, to investigate what happened.

The memo, however, provides important new information and context to the events that have led to a growing controversy in Congress, where Republicans led by Sen. Charles Grassley of Iowa and House Oversight and Government Reform Chairman Darrell Issa have collected extensive evidence that ATF agents vehemently objected to letting the guns walk but were overruled by supervisors, who let hundreds of a weapons a month flow to straw buyers even as violence escalated inside Mexico in 2010.

On Wednesday, Grassley and Issa will release a joint report concluding that Justice officials in Washington were ultimately responsible for letting a well-intentioned gun trafficking strategy go awry.

“The Department’s leadership allowed the ATF to implement this flawed strategy, fully aware of what was taking place on the ground,” their report concludes, according to an early copy obtained by The Beast.

“This hapless plan allowed the guns in question to disappear out of the agency’s view. As a result, this chain of events inevitably placed the guns in the hands of violent criminals. ATF would only see these guns again after they turned up at a crime scene. Tragically, many of these recoveries involved loss of life,” the report concludes.

Democrats on Issa’s committee and defenders of the Justice Department are expected to mount a technical defense, arguing that pressing law enforcement officials on the border to try new strategies did no specifically address letting guns walks, a decision that to date appears to have been made by the U.S. attorneys office and ATF supervisors in Phoenix, along with ATF officials in Washington.