U.S. Attorney General Eric Holder says the effects of a failed investigation into weapons-trafficking on the U.S.-Mexico border will be felt for years to come.

Holder spoke Tuesday before the Senate Judiciary Committee in a hearing on the so-called “Fast and Furious” operation meant to track guns bought in the United States and smuggled into Mexico.

Agents with the U.S. Alcohol, Tobacco, Firearms and Explosives agency have told lawmakers they were ordered by their bosses to stand aside while gun buyers purchased weapons that were suspected to be destined for Mexican drug cartels. The agents said they were told not to arrest the buyers, but instead to track where the purchasers went.

Two of the guns later turned up at the scene of a shoot-out in Arizona that left a U.S. border patrol agent dead.

Holder said Tuesday that any incidence of so-called “gun walking” is unacceptable and must never happen again. He added that the scandal has highlighted the fact that the U.S. is losing the battle to stop the flow of illegal guns to Mexico.

The revelations have outraged some members of Congress, prompting calls for Holder's resignation.