When you give guns to the Mexican cartels, they use them to kill people. That is the finding of an investigation by the Bureau of Alcohol, Tobacco and Firearms, which involved allowing thousands of guns collected through suspicious purchases to walk across the Mexican border. The guns were later used against U.S. Border Patrol agents. You can read more about the scandal from CBS News here.
Ever since Border Patrol agent Brian Terry was killed in December, the "gunwalker" scandal has been slowly growing. Two guns recovered on the scene of the murder had been deliberately allowed by BATF to walk across the border. The Justice Department continues to evade responsibility for this investigation gone wrong, and Senate Democrats seem to have no interest in holding them accountable.
House Oversight Chairman Darrell Issa, R-Calif., has been investigating, and receiving minimal cooperation from the administration. He released this statement upon the indictment of Terry's killer:

“The announcement of an indictment against Manuel Osorio-Arellanes

for the murder of Border Patrol Agent Brian Terry is certainly good news, but leaves critical questions unanswered. The Justice Department still hasn’t said how and why guns purportedly being tracked and monitored by federal law enforcement officials as part of Operation Fast and Furious ended up in the hands of Agent Terry’s killers.  It angers me to think that this death might not have occurred had it not been for reckless decisions made by officials at the Department of Justice who authorized and supported an operation that knowingly put guns in the hands of criminals.  For these officials to imagine that this operation would result in anything other than a tragic outcome was naive and negligent.  Sen. Charles Grassley and I continue to demand accountability as we investigate this matter.”

This scandal is especially striking given all the complaining and the false numbers that President Obama released at the beginning of his administration over the number of U.S.-bought guns being used by cartels in Mexico.