ATF Special Agent John Dodson in 2011. (CBS News)

Two years ago, federal agent John Dodson turned whistleblower and exposed a botched gun operation in Phoenix that led to senior-level resignations, 18 months of congressional investigations and the first vote in history by the House to hold a sitting attorney general in contempt of Congress.

Now, the Bureau of Alcohol, Tobacco, Firearms and Explosives, where Dodson works, is preventing him from publishing a book about the failed gun investigation, dubbed “Fast and Furious,” because the agency says it would hurt morale at the agency.

“This would have a negative impact on morale in the Phoenix [field division] and would have a detremental [sic] effect on our relationships with [the Drug Enforcement Administration] and FBI,” the rejection letter said.

The American Civil Liberties Union came to Dodson’s defense Monday and filed a protest with the ATF, strongly objecting to the agency’s efforts to block Dodson from publishing his book, which has been written, saying the decision violates his “constitutional protections.”

“It was Agent Dodson’s disclosures that helped bring the operational failures at the Phoenix field division to light,” the ACLU wrote in a letter to ATF Deputy Director Thomas E. Brandon. “As a knowledgeable and informed ‘insider’ who was directly involved in Operation Fast and Furious, Agent Dodson will add significantly to the national conversation about gun policy.”

A law enforcement official said that a government-wide ban prevents federal employees from receiving compensation “from any source other than the government for teaching, speaking or writing that relates to the employee’s official duties.” The official said the ATF is conducting a review to determine whether Dodson would be revealing any information that is “law enforcement sensitive.” If not, the official said, Dodson could publish his book without receiving compensation.

Sen. Charles E. Grassley (R-Iowa) and Rep. Darrell Issa (R-Calif.), two persistent critics of the ATF, wrote a foreword for the book.

“This isn’t the first time somebody from the ATF or another government agency has written a book,” Grassley said. “Just because the ATF leadership doesn’t like the content of the book doesn’t mean they should be able to prevent the author from giving his side of the story.”

During the gun-trafficking operation run by Phoenix special agents between late 2009 and early 2011, the ATF lost track of more than 2,000 guns that investigators were monitoring as they were sold to traffickers suspected of arming Mexican drug cartels. The operation to link guns to a cartel fell apart after two of the guns being tracked were found at the scene of a shootout that killed U.S. Border Patrol Agent Brian Terry.

It was soon after Terry’s death that Dodson came forward and told House Republican lawmakers and staff members about the secret operation.

Dodson later testified that on his first day working undercover in Phoenix, he sat in a car with another special agent and watched a suspected gun trafficker buy 10 semiautomatic rifles from a suburban Phoenix gun store. The other agent and Dodson, who had been an ATF agent for seven years, followed the man to the house of another suspected trafficker. They wanted to move in and seize the guns.

But Dodson said that when they called their supervisor and asked for the order to “take him,” the message was clear. They were to let the guns go.

It was part of a strategy in Phoenix that allowed ATF agents to follow the paths of guns from buyers who bought them illegally, known as “straw purchasers,” through middlemen and into the hierarchy of a drug cartel. In this case, ATF officials said they were trying to make a high-impact case showing that the firearms were going to the upper ranks of the powerful Sinaloa drug cartel.

But Dodson and some of his fellow agents were upset about the huge volume of firearms being let go — a tactic they called “gun-walking.” The agents said they feared the worst, and on Dec. 14, 2010, it happened: Terry was patrolling Peck Canyon in the Arizona desert when he and other agents got into a gunfight with suspected illegal immigrants and was fatally shot.

Two AK-47 semiautomatic rifles were found nearby. The guns ended up being on the list of firearms that were bought at the Phoenix gun store by one of the gun traffickers whom the ATF was monitoring.

The revelations and congressional investigation embarrassed the ATF, and led to the resignation of Arizona U.S. attorney Dennis Burke and the reassignment of ATF acting director Kenneth E. Melson. Attorney General Eric H. Holder Jr. was held in contempt of Congress after the Justice Department refused to turn over to Congress certain documents about internal deliberations after Fast and Furious ended.