The families of two federal agents shot more than a year ago on a Mexican highway have renewed their demand 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 twin sister of agent Victor Avila on Friday in Brownsville to remind the public that the government has not answered their questions.

Mary Zapata, the agent's mother, believes guns used in the attack came from a government operation similar to Fast and Furious in Arizona. That program allowed guns purchased illegally in Arizona to be smuggled to Mexico in hopes of tracking them to higher-ranking criminals.