Police in Mexico have arrested the fugitive mayor of the town of Iguala, where 43 students went missing in September, along with his wife.
Jose Luis Abarca was detained by federal officers in the capital, Mexico City, a police spokesman said.
Mexican officials have accused Mr Abarca of ordering police to confront the students on the day of their disappearance on 26 September.
Eyewitnesses described seeing them being bundled into police cars.
Mexican President Enrique Pena Nieto praised the work of prosecutors and the security forces.
"I hope that [the couple's] arrest will clarify many points being investigated by federal prosecutors," Mr Pena Nieto said.
Mexican officials had issued an arrest warrant for Mr Abarca and his wife, Maria de los Angeles Pineda, after Iguala police officers said they had received an order from the mayor to intercept the students.
The officers said they had been told to stop the students from interrupting a speech given by Ms Pineda in Iguala on that day.
"This was the missing piece. This arrest will help us find our kids," Felipe de la Cruz, a parent of one of the missing told Milenio television.
The students, from a nearby teacher training college, had travelled to Iguala to raise funds and protest.
They have not been seen since. A search has uncovered a series of mass graves in the area, but initial tests suggested they were not those of the students.
Since then, more bodies have been uncovered and officials have cast doubt on the accuracy of the initial tests.
More forensic tests are currently being carried out.
Timeline: Iguala disappearance
26 Sept: Students from a teacher training college in Ayotzinapa travel to Iguala to protest and raise funds
Night of 26 Sept: Police stop the students, 6 people are shot dead, 43 students disappear
30 Sept: Iguala mayor Jose Luis Abarca asks for leave from his post, which is granted
4 Oct: Mass graves are found near Iguala containing 28 bodies
19 Oct: Federal police are deployed to Iguala and replace the municipal force
22 Oct: Mexico's prosecutor general says an arrest warrant has been issued for Mr Abarca, his wife and the town's police chief
23 Oct: Guerrero state governor Angel Aguirre resigns
29 Oct: President Enrique Pena Nieto meets the relatives of the missing students and promises a "renewed search plan"
4 Nov: Mr Abarca and his wife are arrested in Mexico City
Mr Abarca and Ms Pineda were arrested in a house they had rented in Iztapalapa, a neighbourhood of Mexico City. They did not resist arrest.
They have been taken for questioning. Officials hope they will be able to shed light on the whereabouts of the students.
The events of 26 September have shocked Mexicans and have led to mass protests demanding that the authorities do more to find the missing students.
The 43 were part of a larger group which had gone to Iguala to protest against what they said were discriminatory hiring practices.
The students all attended a teacher training college in Ayotzinapa with a history of left-wing activism, and their presence in Iguala raised alarm bells with the local authorities.
When the students boarded busses to return to their college, they were stopped by police - allegedly on the orders of Mayor Abarca.
The officers opened fire and killed three students and three people in nearby vehicles.
They stopped one busload of students trying to flee and took them to a local police station.
According to police officers detained as part of the investigation, they then handed the students over to a local drugs gang.
The gang's leader, who has also been arrested, says he ordered the students be "made to disappear", after having been told they belonged to a rival gang.
However, he did not specify further what happened to them.
The gang leader also accused Ms Pineda of being "the main operator of criminal activities in Iguala".
The relatives of the missing students said on Monday that "no progress" had been made in the search for the 43 and expressed their anger over the slow pace of the investigation.
The governor of the state of Guerrero, where Iguala is located, resigned last month over the disappearances.