A member of a gun-smuggling ring linked to the shooting death of a U.S. immigration agent in Mexico pleaded guilty here Thursday to federal charges involving the illegal purchase of firearms.

The plea comes at a time when some Republicans in Congress are raising concerns that the agents in charge of the case in Texas might have intentionally allowed suspected smugglers to acquire guns, as other federal agents did in two operations in Arizona. In one Arizona sting, known as Fast and Furious, federal investigators lost track of some guns that were then used to commit crimes in the U.S. and Mexico.

In the Texas case, 28-year-old Ranferi Osorio of Lancaster, Texas, told a federal magistrate judge that he conspired with a crew of seven other men to buy guns in violation of federal firearm-reporting laws. He also admitted to possessing weapons with scratched-out serial numbers.

His brother Otilio, who is set to plead guilty to the same charges next week, bought the Romarm-Cugir Draco 7.62 pistol that Mexican drug-cartel members used in a February ambush that killed Jaime Zapata, the Immigration and Customs Enforcement officer, on a highway between Mexico City and Monterrey.

A spokesman for the Dallas office of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, which conducted the Osorio investigation, declined to comment on the case because it is still open. In the past, the head of the office has said that his agents didn't intentionally allow any weapons to cross the border.

Events in the Osorio case took place in 2010 and early this year.