The head of the criminal investigation agency for the Attorney General's Office, whose dismissal had been demanded by the families of 43 college students who disappeared two years ago, resigned Wednesday.

Tomas Zeron's departure had been a rallying cry for the parents of the youths who haven't been seen since being taken away by police in the town of Iguala in southern Guerrero state. Zeron was at the center of the investigation that has failed to determine the whereabouts of the students, who were allegedly handed over to a drug gang and slain.

No reason was given for Zeron's resignation. A brief statement from the Attorney General's Office said only that Attorney General Arely Gomez wished him luck.

Zeron oversaw not only the agency's investigators, but also its forensic work.

The government's investigation into the students' disappearance has been criticized within Mexico and by international experts for focusing on an early theory that the students' bodies were incinerated at a dump site rather than on investigating other leads. The case has become an embarrassment for the administration of President Enrique Pena Nieto.

Two independent, international teams of experts cast doubt on what a former attorney general dubbed at the time the "historic truth" of what happened to the students.

Many of the suspects rounded up in the investigation have complained they were tortured into backing the government's version of what happened. Court documents obtained by The Associated Press in May showed that 10 of the suspects described similar treatment at the hands of authorities and some even said they were given planted evidence or prefabricated stories.

In April, the students' families called for Zeron's firing over missteps in the investigation. They called for Zeron himself to be investigated for "crimes related to obstruction of justice."

The Attorney General's Office, which oversees the investigative agency led by Zeron, said in a statement at the time that it had opened an investigation through its internal affairs unit.

On Wednesday, the families' representatives did not immediately respond to calls for comment, but released a statement saying the families would hold a news conference Thursday to address Zeron's resignation.

Experts sent by the Inter-American Commission on Human Rights had criticized Zeron for not clearly documenting how burned bone fragments — the only physical evidence of the students — were found in a river near the dump where the government says they were disposed of after the fire.

Zeron acknowledged that some bone fragments had been registered as found a day before they actually were. One fragment was later tied by DNA testing to a missing student. The experts said Zeron visited the river on Oct. 28, 2014, with one of the suspects who complained of torture. The government said the bone fragments were found there the next day.

The students attended the Rural Normal School of Ayotzinapa, a teachers college. They were in Iguala on Sept. 26, 2014, to hijack buses to use for transportation to a rally in Mexico City. They were attacked on the buses by local police and allegedly handed over to members of the Guerreros Unidos cartel.