Relatives of the 43 missing students from Ayotzinapa take part in a protest in Chilpancingo, Guerrero State, Mexico on September 26, 2015

Two Mexican federal police officers possibly participated in the disappearance of 43 students, the National Human Rights Commission said Thursday, implicating national agents in the 2014 case for the first time.

The announcement adds a new twist to a probe that has come under fire from international human rights groups and independent investigators.

Jose Larrieta Carrasco, a commission official investigating the case, said the authorities should now look into a "new route in the disappearance" of the students.

The attorney general's office said it would "deepen" this "line of investigation" following the commission's announcement.

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 the southern city of Iguala on September 26-27, 2014.

But the governmental rights commission said it found an eyewitness who saw two federal agents near Iguala's courthouse, where municipal officers had stopped a bus with 15 to 20 students on board.

The commission also said another local police department, from the town of Huitzuco, had a previously unknown role in the disappearance.

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 tires of the bus that stopped near the courthouse, prompting the students to toss rocks at the police.

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 twisting saga, as authorities have maintained that suspects told investigators that the students were killed in the nearby town of Cocula.

The commission said 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 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.

- Investigation questioned -

The attorney general's office said it was providing protection to the eyewitness cited by the commission as well as the person's family.

Prosecutors declared last year that police officers from Iguala and the neighboring town of Cocula abducted the students and delivered them to the Guerreros Unidos drug cartel.

The gang then killed the students, incinerated their bodies at a garbage dump in Cocula and dropped the remains in a nearby river.

But experts from the Inter-American Commission on Human Rights who conducted an independent investigation said there was no scientific evidence the 43 students were incinerated at the dump.

The case is considered the biggest challenge faced by President Enrique Pena Nieto's administration, prompting protests and causing his approval rating to drop.