In some areas, the criminals operated as virtual shadow governments, particularly along the border with the United States.

But Vivanco said the likelihood that police and military were involved in at least some of the crimes increases the obligation of the Pena Nieto government to prosecute. To do otherwise, he said, would just encourage more abuses.

Vivanco called for “exemplary punishment” for soldiers and police found guilty of crimes with “sentences proportional to the atrocities that have been committed by public forces in a war against drug traffickers in which everything was permitted. This is the result of a war without any controls.”

Building cases from long-cold crimes will not be easy, he said, especially when “officials manipulated crime scenes, fabricated evidence to clear themselves and implicate others.”

Judicial investigators are often poorly trained, unable to carry out simple tasks for building a criminal case, added Nik Steinberg, a Mexico researcher for Human Rights Watch. In one case, he said, “investigators spoke to the wives of disappeared men, and then took DNA samples from the wives. . . . This is an example of the capabilities of the investigators they are sending.”

Vivanco noted that citizens placed more than 5,000 complaints of abuses against naval and army forces during the Calderon government. “How many led to conviction? Thirty-eight,” he added.

Recent years mark the bloodiest chapter in Mexico’s history since its national revolution, which took more than 1 million lives between 1910 and 1920.

Other Latin nations have known intense periods of bloodshed as well. A period between 1948 and 1958 in Colombia, known as La Violencia, left 200,000 people dead.

Civil wars in Central America also left vast tolls. A Guatemalan insurgency that began in 1960 and lasted until 1996 triggered bloodshed that left 200,000 Guatemalans dead and 50,000 missing. Civil wars in Nicaragua and El Salvador took 60,000 and 75,000 lives, respectively.

Calculating the disappeared in Mexico draws particular challenges because of the nation’s proximity to the United States and the tendency of some Mexicans to cross the border secretively, even slipping away without informing family members.

Some migrants – especially those from Central America – have fallen into the grasp of organized crime in northern Mexico and suffered mass killings. In August 2010, Mexican marines found the bodies of 72 migrants. Less than a year later, they found mass graves containing 193 bodies.

Verastegui said he believes the figure of 27,000 missing may actually be shy of the real number.

“In a great majority of the cases, people don’t dare denounce the crime,” he said, “because the authorities are the ones linked to organized crime or actually participated in the disappearances. The families are afraid.”