Updated 11:55 p.m. ET
A top figure in the gunwalking controversy at the Bureau of Alcohol, Tobacco and Firearms (ATF) is now cooperating in the investigation.
Sen. Charles Grassley (R-Iowa) made that disclosure today in a letter to ATF's acting director Kenneth Melson.Read Grassley's letter
In the letter, Sen. Grassley warns that any attempt to retaliate against the cooperating official, Assistant Special Agent in charge of ATF's Phoenix Division George Gillett, is unlawful. Prior to Sen. Grassley's letter, Gillett had already told his supervisors, through his attorney, that he was cooperating.
Sen. Grassley also says the apparent efforts of ATF executives to stop employees from speaking with members of Congress and their staff is of "grave concern."
"Without such direct, unfiltered communications, Congress would still be unaware of, and unable to inquire about, the serious allegations involving the death of Border Patrol Agent Brian Terry and the sales of weapons to known and suspected gun Traffickers," reads the Sen. Grassley letter.
Rep. Darrell Issa (R-CA), chairman of the House Oversight committee, has joined Sen. Grassley in investigating the gunwalking scandal exposed by CBS News. Rep. Issa recently issued ATF a subpoena for information when that agency failed to respond to information and document requests from his office and Sen. Grassley's.
ATF Special Agents including John Dodson, Rene Jaquez and the former head of ATF operations in Mexico Darren Gil have all spoken to CBS News to blow the whistle on the effects of ATF's alleged gunwalking scheme. They say ATF allowed thousands of assault rifles and other weapons to "walk" across the border into the hands of Mexico's drug cartels. ATF apparently hoped that letting the guns hit the streets and then seeing where they later ended up would help them take down a major drug cartel. That never happened. Instead, the guns were used in many crimes in Mexico. Two of the weapons were found at the murder scene of Border Patrol Agent Brian Terry. Officials are investigating possible connections to the murder of ICE Agent Zapata. The two federal agents were gunned down two months apart.
After ATF whistleblower Dodson spoke to Grassley in January, he disclosed the contact to his supervisors. Sen. Grassley says Dodson was then subjected to intimidation. Dodson "was immediately questioned about the content of those communications. I was concerned about that because forcing an employee to reveal the details of such communications would intrude on the integrity of the Congressional inquiry and offend the comity between the Branches that flows from the separation of powers under the Constitution," says Sen. Grassley in today's letter.
When contacted today by CBS News, neither the Justice Department nor ATF had any immediate comment and have refused our repeated interview requests.
In an interview with Univision last month, President Obama has said neither he nor Attorney General Eric Holder approved the operation. "There may be a situation here which a serious mistake was made and if that's the case then we'll find out and well hold somebody accountable," Mr. Obama said. Holder is in charge of the Justice Department which oversees ATF. Holder has asked the Inspector General to look into the allegations.
Sen. Grassley says the Justice Department, ATF and the State Department all failed to answer information requests from Congress.
All of Sharyl Attkisson's articles, blogs and videos can be found in one place at cbsnews.com/sharylattkisson