While there is little argument that the violence in Mexico caused by the country’s drug war has reached epidemic levels, data over how much of that spillover has reached the U.S. varies greatly.

Though there are discrepancies in numbers – data differs from study to study – the violence seems to be contained in Mexico, though there are concerns that it could endanger American citizens, according to a Government Accountability Office (GAO) report.

“Law enforcement agencies have few efforts to track spillover crime,” the GAO report stated. “Law enforcement agencies have varying concerns regarding the extent to which violent crime from Mexico spills into southwest border communities.”

The GAO study found that officials in 31 of 37 state and local agencies said that they have not observed violent crime from Mexico regularly spilling into their area, but that 33 of these agencies were at least somewhat concerned for the safety of their personnel or residents.

Drug war violence has remained mainly in Mexico, but has recently subsided in such hotspots as Ciudad Juárez, the violent town that sits directly across the El Paso, Texas border. An estimated 70,000 people have died in drug-related violence since former Mexican President Felipe Calderón declared war on the country’s drug cartels in 2006.

Besides the death toll, Mexico’s cartels – and with them, violence – have spread from certain regions along the U.S.-Mexico border to other parts of the country once free of violence, including resort areas such as Acapulco and metropolitan hubs like Monterrey and Guadalajara.

Spillover violence, however, took center stage following the murder of Border Patrol agent Brian Terry in 2010.

Terry's death was a catalyst for uncovering the U.S. government's botched gun walking investigation known as "Operation Fast and Furious."

Two rifles bought by a gun-smuggling ring that was being monitored by the government's investigation were found at the shooting scene. The two guns were bought by 25-year-old Jaime Avila Jr., who pleaded guilty in federal court to conspiracy and dealing guns without a federal license and was sentenced to nearly five years in federal prison.

Operation Fast and Furious was launched in 2009 to catch trafficking kingpins, but federal agents lost track of about 1,400 of the more than 2,000 weapons in the operation – including AK-47s and other high-powered assault rifles.

Some of the guns purchased illegally with the government's knowledge were later found at crime scenes in Mexico and the U.S.

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