Six months ago, several agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives stood before Congress to testify about the details of a U.S. government program that armed Mexico's largest drug cartel with thousands of assault rifles. The administration denied it at the time and questioned the agents' integrity. The men were nervous and scared. They said they feared for their careers, their reputation and their families. Witnesses are sworn in at a hearing held by the House Oversight and Government Reform Committee on "Operation Fast and Furious: The Other Side of the Border," on Capitol Hill in Washington, July 26, 2011. Border Patrol Agent Brian Terry was killed on Dec. 14, 2010. Weapons found at the scene were linked to Operation Fast and Furious, the ATF program that let guns be purchased illegally and cross the border. Several have now reportedly been tied to crimes in the U.S. "Any attempt to retaliate against them for their testimony today would be unfair, unwise and unlawful," Sen. Charles Grassley, R-Iowa, warned the Department of Justice. Read more: 'Fast And Furious' Whistleblowers Struggle Six Months After Testifying Against ATF Program | Fox News This just ain't right.