Mexican authorities have expanded the search for 43 students who vanished after they were abducted by corrupt police in 2014, a government official said.
Relatives of the students have questioned the conclusions of the investigation, urging authorities to keep looking for them despite claims they were killed and incinerated at a landfill by a drug gang in southern Guerrero state.
Roberto Campa, the deputy interior minister for human rights, told Agence France Presse that the authorities are looking into allegations that the students were taken to other locations.
Campa said, "The big question is, what did they do with the boys after making them disappear?"
The official said authorities conducted searches in the central states of Mexico and Puebla in recent weeks.
"The effort that has been done in recent weeks have been extraordinary," Campa said.
In total, 600 search operations have taken place since the students disappeared in the city of Iguala in September 2014, Campa said.
"We are working exhaustively on four lines" of investigation, Campa said in Tuesday's interview.
Attorney General Arely Gomez will discuss the new lines of investigation when she meets with parents of the students on Thursday.
Her office created a new special unit for the search of missing people in October - a key demand from the parents, whose anguish has inspired protests over President Enrique Pena Nieto's handling of the tragedy.
Former attorney general Jesus Murillo Karam had concluded that police handed the students over to the Guerreros Unidos drug gang, which killed them and incinerated their bodies at a garbage dump.
But independent investigators from the Inter-American Commission on Human Rights have insisted that there was no scientific evidence that the students were incinerated at the Cocula landfill.
The attorney general's office has agreed to form a new group of experts to conduct a new investigation at the site of the alleged fire.
Campa said that the landfill remains among four lines of investigation.
One hypothesis comes from the testimony of a suspected hitman who told investigators that 13 students were taken to a hill outside Iguala, where nine were shot and burned while four others were tied up, beaten and left unconscious.
Another claim comes from a different suspect, who said a group of students were taken to a house in a district of Iguala known as Pueblo Viejo.
In October 2014, state prosecutors said a suspect confessed to killing and incinerating 17 students in Pueblo Viejo.
The fourth theory derives from statements from the driver of a bus that the students had hijacked before disappearing.
The driver said he overheard police say "those from Huitzuco are coming" for the students. Huitzuco is a crime-plagued town near Iguala where the Guerreros Unidos have battled the rival Los Rojos gang.