Federal police may have been involved in the abduction and murder of 43 students in Guerrero state two years ago, Mexico has said for the first time.
The admission comes after its national human rights commission found a witness who came forward with evidence.
The witness reported that two federal police and a third municipal police force were present when the students were taken off a bus and may have even participated in their disappearance, Jose Larrieta Carrasco, the commission member leading the case, said.
Thursday's announcement added a new twist to a probe that has come under fire from international human rights groups and independent investigators.
The attorney general said the students had been taken by the corrupt officers and given to a drug cartel, who killed them and dumped their bodies, adding that the announcement would "deepen" this "line of investigation".
International investigators say there is no evidence to support this.
A federal police statement said the allegations were not new and that all officers present in the region that night gave statements, but that "for the moment" no wrongdoing was found against any agents.
Prosecutors have already charged municipal police officers in connection with the mass abduction in Iguala, a city in Guerrero, on September 26-27, 2014.
The bus was one of five that around 100 students had seized that night to use for a future protest. Iguala police officers opened fire on the buses before the students disappeared.
The commission said the police fired on the tyres of the bus that stopped near the courthouse, prompting the students to toss rocks at the police.
READ MORE: Mexico one year after the kidnapping - 'I saw it all'
The officers bundled the students into several patrol vehicles, including three from Huitzuco.
When the federal officers arrived, they asked what was going on.
An Iguala officer said the students would be sent to Huitzuco, where "the boss" - possibly a drug cartel member - would "decide what to do with them", the commission said.
The federal officers responded, "Ah, OK, that's good," and allowed the local police to take the students away.
Huitzuco would be a new location in the saga, as authorities have maintained that suspects told investigators that the students were killed in the nearby town of Cocula.
The commission said that there was enough evidence to "presume the participation of members of the Huitzuco municipal police and two federal police officers" in the disappearance, adding that it has the name of one of the two federal agents, which it gave to prosecutors.
The commission also said that a soldier on a motorcycle took pictures of the incident and then left. Families of the victims have called for an investigation into whether the military had a role in the case, but the army denies any wrongdoing.
In some parts of Mexico, companies have been coerced into not reporting crimes by drug gangs who threaten to attack their vehicles, offices or employees.