The head of the Mexican agency investigating the disappearance of 43 college students has resigned.

Tomas Zeron was in charge of the criminal investigation agency for the Attorney General's Office, and his dismissal had been demanded by the families of the students, who have not been seen since being taken away by police in Iguala in southern Guerrero state.

Mr Zeron was at the centre of the investigation that has failed to determine the whereabouts of the students, who were allegedly handed over to a drug gang and killed.

No reason was given for the resignation of Mr Zeron, who oversaw not only the agency's investigators, but also its forensic work.

The government's probe into the students' disappearance has been criticised within Mexico and by international experts for focusing on an early theory that their bodies were incinerated at a dump 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 Mr Zeron's firing over mistakes in the inquiry. They called for him to be investigated for "crimes related to obstruction of justice".

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

The families will hold a news conference on Thursday to address Mr Zeron's resignation.

Experts sent by the Inter-American Commission on Human Rights had criticised Mr 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.

Mr 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 Mr Zeron visited the river on October 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 September 26 2014, to hijack buses to use for transport 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.