Kenneth Melson’s acknowledgment is the first by any senior ATF leader that confirms some of the criticism that congressional Republicans have been leveling at Operation Fast and Furious. The objections have resulted in congressional hearings and an inquiry by the US Justice Department’s inspector general.

Melson’s comments were quoted in a letter Monday to Attorney General Eric Holder from two Republican lawmakers, Sen. Chuck Grassley and Rep. Darrell Issa. Melson spoke during private questioning this month by Senate and House investigators working for the two Republicans. Their letter contained excerpts from Melson’s testimony.

The operation was designed to track small-time gun buyers up to major weapons traffickers along the Southwest border. Critics estimate that 1,800 guns targeted in the operation are unaccounted for, and about two-thirds of those probably are in Mexico.

Fast and Furious is one of thousands of US law enforcement investigations launched since 2006 aimed at stemming the flood of weapons across the US border with Mexico.

At a congressional hearing a month ago, three ATF agents said they were ordered repeatedly to step aside while gun buyers in Arizona walked away with AK-47s and other high-powered weaponry headed for Mexican drug gangs. So far, 20 small-time gun-buyers have been indicted, but the investigation still is under way.

Melson told the congressional investigators that after the criticism emerged he assigned a task force of agents to review the operation and he personally read all the operations’ investigative reports that pertained to one defendant.

“I read through those and found ROIs (reports of investigation) that indeed suggested that interdiction could have occurred, and probably should have occurred, but did not occur,” Melson said.

Melson said the ATF gave the reports about the failure to interdict guns in that instance to Justice Department superiors “because to me that was a smoking gun” and “we really needed to look at the rest of this particular case.”

Melson also provided some support for complaints by Grassley and Issa that the Justice Department has been slow in producing documents the lawmakers have demanded.

“I think they were doing more damage control than anything” at the Justice Department, Melson said, according to one of the excerpts in the letter from Grassley and Issa to Holder.

In his statements to congressional investigators, Melson also said he understood that some potential defendants in the investigation may not be indictable because they are working as informants with other government agencies. Melson said he raised the issue with the deputy attorney general and with the Justice Department’s inspector general.

The Fast and Furious operation targeting high-level weapons merchants as well as street-level buyers was an ATF attempt along the Arizona border to respond to its many critics there.

As recently as last November, the Justice Department’s inspector general criticized some ATF managers for discouraging field personnel from conducting complex conspiracy investigations that target higher-level members of trafficking rings.