Lt. Col. Guillermo Bruno Laborda was upset he didn't get the promotion to full colonel in the Argentine army he felt he deserved. So he wrote an angry letter to the army brass last month detailing the "meritorious" acts of his 28 years of military service.

As a young lieutenant in the late 1970s, he wrote, he personally had executed prisoners, and then set their bodies on fire, just as his superiors had ordered. He had shot a young mother a day after she delivered her baby, and then tossed the woman's body into a hole and set it on fire too.

During Argentina's "dirty war" against dissidents and urban guerrillas, these "were considered true and unavoidable acts of service," he wrote, and all the emotional pain he had endured because of them should be taken into account in the decision on his promotion.

Bruno Laborda's chilling letter -- complete with the final words of many of his victims -- marks the first time the military has made public an active-duty officer's confession to his role in extrajudicial executions during Argentina's bloody years of dictatorship and repression. Not long after he submitted it, the military high command had him arrested.

Although reported in the Argentine media with little fanfare, the case demonstrates an important shift in the country's military culture. It is widely believed here that officers seeking promotion routinely made arguments similar to those made by Bruno Laborda but that they were kept secret by an unwritten code of military silence.

Now nearly 100 current and former military men are in jail in Argentina, more than at any other time since 1987, when dozens were detained after a failed coup. The majority have been imprisoned since May 2003, when President Nestor Kirchner came to office promising to aggressively prosecute the human rights crimes of the past.

"Those military men who have been implicated in criminal acts, and found culpable by the justice system, will be automatically eliminated from the force," Gen. Roberto Bendini, the army chief, said after Bruno Laborda's arrest.

Confronted with the lieutenant colonel's letter, Bendini said, "We had no choice but to file the appropriate charges. And have no doubt that we will continue to do so when presented with this type of evidence."

On March 24, the 28th anniversary of the country's 1976 military coup, Gen. Bendini participated in another, more symbolic break with Argentina's past: He removed the portraits of two members of the ruling junta, generals Jorge Videla and Reynaldo Bignone, from their place of honor at the Military College in Buenos Aires.

Obtained and published this month by the Buenos Aires daily newspaper Pagina 12, Bruno Laborda's letter offers a horrific vision of the crimes committed during the regime of the generals.

In 1977, as a 23-year-old lieutenant just a few months out of the military academy, he was assigned to the 3rd Army Corps, based in Cordoba, in central Argentina. Soon afterward, he was ordered to "actively participate in the physical elimination of [a prisoner] accused and condemned as a guerrilla," although he never learned who, if anyone, had pronounced the sentence.

More executions followed, the officer, who is now 50, wrote. In 1978 he was part of a firing squad that killed a young mother who had been brought to Cordoba's military garrison in an ambulance, a day after giving birth. (Children born to detainees routinely were turned over to military families in secret adoptions.) "On her knees and blindfolded, she received the impact of more than 20 bullets of various caliber," Bruno Laborda wrote. "I never found out what happened to the baby boy or girl."

That killing, like all the others, traumatized him. "The continuous weeping, the very odor of adrenaline that comes from those who can feel their end coming, their desperate cries begging us that if we were really Christians we would swear we weren't going to kill them, was the most pathetic, agonizing and saddest thing I ever felt in my life and I will never forget it," he wrote.

Such descriptions have been rare in democratic, post-military rule in Argentina, especially after then-President Raul Alfonsin in 1986 and 1987 granted members of the military amnesty for all crimes and immunity from further prosecutions.

In the past year, however, Argentine judges have found creative strategies to circumvent the amnesty, which is expected to be overturned soon by the country's revamped Supreme Court.

Bruno Laborda is being held in a military prison in Buenos Aires and has been ordered to appear before a federal judge in Cordoba investigating killings attributed to the 3rd Army Corps.

In his letter, Bruno Laborda pointed out that other officers who had participated in the executions had been promoted. He added that, as a 23-year-old, he had sought and received absolution for the killings from a priest who told him he would be "rewarded for destroying the enemies of Christ."