U.S. Attorney General Eric Holder has faced a new round of Republican criticism over a controversial tactic in which U.S. authorities allowed gun buyers to move arms into Mexico, destined for drug cartels.

In testimony to Congress Thursday, Holder said “Operation Fast and Furious” was flawed and allowing arms to “walk” under any circumstance is unacceptable.

Republicans charge that the Justice Department has not provided enough information to them.

The head of the House Committee on Oversight and Government Reform, Darrell Issa, has accused Holder of protecting staff, deceiving the public and obstructing the panel's investigation.

Holder told lawmakers on the committee Thursday that he never authorized the operation, and ordered it to stop after he found out about it.

U.S. authorities lost track of hundreds of weapons they were supposed to be monitoring. Many weapons were eventually recovered from crime scenes. Two were found at the scene where U.S. border agent Brian Terry was shot and killed in 2010.

Holder said the Justice Department is building its case and is close to making announcements regarding arrests and holding those responsible for the agent's death and the flawed operation.