“That only gets you so far when the reality on the ground is so different,” she said.

In addition, Ms. Meyer said, Congress had strengthened the conditions for the 15 percent of police and military aid since 2013, asking for evidence that the Mexican government was moving forward on eliminating torture and solving disappearances.

Repeated cases over the past year have cast a harsh light on the Mexican government’s failure to rein in the abuses of its security forces and to solve cases.

In particular, the disappearance of 43 students in the southern state of Guerrero in September 2014 has come to represent the yawning gap between Mexico’s public commitment to respecting human rights and the turmoil in the states subsumed in drug violence.

The government of President Enrique Peña Nieto said that the students had been attacked and abducted by corrupt local police officers, who handed them over to a local drug gang called Guerreros Unidos. Hit men from the drug gang confessed that the young men, who were training to become rural teachers, were transported to a remote garbage dump, killed and burned to ashes on a diesel-fueled pyre of tires and wood.

But an independent investigation by a group of experts appointed by the Inter-American Commission on Human Rights concluded last month that the students could not have been incinerated at the dump as the government described. The group also found that the military and the federal police were aware of the initial police attack on the students and did nothing to halt it.

In another high-profile case, soldiers killed 22 people, suspected of being members of a drug gang, in what the military and civilian authorities first described as a shootout last year. But Mexico’s national human rights commission later found that at least 12 of them had been executed.

Prosecutors initially charged seven soldiers and one lieutenant in the killings. After judicial rulings, only three soldiers remain to face trial in the case.