The families of two U.S. immigration agents shot more than a year ago on a Mexican highway renewed their demand Friday that the U.S. government explain the decisions that put them there and answer questions about how guns purchased in the U.S. fell into the hands of their attackers.

The parents of slain Immigration and Customs Enforcement agent Jaime Zapata joined with the sister of agent Victor Avila to press the government on whether an operation similar to one in Arizona that allowed illegally purchased guns to be smuggled into Mexico in hopes to tracking them to higher-ranking criminal figures was responsible for the guns used in the attack. Two of the weapons have been traced to illegal purchases in Texas.

The families spoke in Zapata's hometown of Brownsville, one day after the U.S. House voted to hold Attorney General Eric Holder in contempt for not providing documents relevant to an investigation of so-called Operation Fast and Furious in Arizona.