Four senior Mexican military officers walked free from prison Friday after they were cleared of charges that they protected a drug cartel, putting a spotlight on the controversial use of anonymous witnesses.

As they emerged at dawn from a maximum security prison in Mexico state, the officers accused the previous administration of Felipe Calderon, who was president from 2006 to 2012, of abusing its power by jailing them.

Authorities ordered late Thursday the "absolute and immediate" release of the military officials after the attorney general's office dropped the charges, citing a lack of evidence in a case accusing them of links to the Beltran Leyva cartel.

Analysts say the case was emblematic of the Calderon administration's reliance on so-called protected witnesses, who included members of drug cartels and whose testimony, they say, was used to sideline political rivals.

Generals Roberto Dawe, Ricardo Escorcia and Ruben Perez, along with Lieutenant Colonel Silvio Hernandez, had been in prison since May 2012.

A fifth officer, Major Ivan Reyna, was also cleared of the charges, but he remained in prison due to a separate fraud case.

"I am very upset, I feel aggrieved," Perez told reporters as family members held the arms of the freed officers. "What happened was vile, and it's good that justice is being rendered."

Their release came two months after a former deputy head of the army, retired general Tomas Angeles Dauahare, was freed also due to a lack of evidence in the high-profile case.

"In the end, the one who put us in prison was the previous president and his attorney general (Marisela Morales)," Escorcia said, calling the case "an abuse of power."

The arrest of the military officers came in the last months of Calderon's presidency, which was marked by a massive troop deployment across the country to combat drug cartels.

Jose Gabriel Baeza, a lawyer for two of the officers, told AFP that the officials had "impeccable careers" in the military and that the accusations against them were based on the testimony of two gang members who were in the witness protection program.

President Enrique Pena Nieto, who took office in December, is changing the security strategy to focus on reducing the drug-related violence that has left 70,000 people dead since 2006. But he says troops will remain on the ground until the murder rate drops.

"The previous administration abused the use of the protected witness," Arturo Argente Villareal, director of the law department at the Monterrey Institute of Technology, told AFP, adding that the new government appears to be dropping the use of such testimony.

"The protected witness was used as an instrument to attack or eliminate political enemies," he said, noting that Angeles Dauahare was seen as a contender to become defense chief under Pena Nieto.

Javier Oliva, a defense expert at the National Autonomous University of Mexico (UNAM), said the case was likely retribution after Angeles Dauahare "questioned" Calderon's anti-cartel strategy at an event that featured then presidential candidate Pena Nieto.

"I think it was political retaliation against Angeles and, by extension, the other generals, in order for the president to reaffirm that he is the commander of the armed forces and that, even though they're retired, they can't move to support anybody," Oliva said.