Mexico came under growing international pressure Tuesday to solve the disappearance of 43 students who vanished after they were attacked by police linked to a drug gang.
The United States and the Organization of American States joined appeals for the country to find the young men, who were last seen 10 days ago in Iguala, a city in the violence-plagued southern state of Guerrero.
Fears over their fate rose over the weekend after authorities found a mass grave up a steep hill outside Iguala containing 28 unidentified and badly burned bodies.
Two gang hitmen told investigators they had executed 17 of the 43 young men and dumped them in pits. But authorities say it will take at least two weeks for DNA tests to confirm the victims' identities.
"This is a troubling crime that demands a full, transparent investigation," said US State Department spokeswoman Jen Psaki. "The perpetrators must be brought to justice."
OAS Secretary General Jose Miguel Insulza called for "the clarification of the murders that have brought grief not only to the Mexican people, but to all the countries of the Americas."
While authorities say the motive remains under investigation, Guerrero's governor said one theory is that the police attacked the students because the mayor's wife was concerned they would disrupt a speech she was giving that day.
- Local police disarmed -
Mexican President Enrique Pena Nieto deployed hundreds of federal police to Iguala on Monday to take over security and disarm the municipal police force.
The attorney general's office dispatched 30 investigators, criminologists and forensic experts to crack the case, which could rank among the most horrific in a drug war that has left 80,000 people dead and 22,000 missing since 2006.
Authorities say the municipal police and its gang allies shot at buses carrying the students on the night of September 26. Several students, from a teacher training college whose radical students often commandeer buses, were later seen bundled into patrol cars.
Another bus carrying a football team was attacked outside the city. In all, six people, including three students, were killed before the 43 went missing.
Authorities have detained around 30 people in the case, including 22 Iguala police officers, while the mayor and city security chief are on the run.
The rest of the police force was hauled to a military base in central Mexico on Tuesday to undergo evaluations while investigators check if their guns were used in any crimes.
As scores of officers in civilian clothing were put on buses, their wives tearfully claimed their innocence.
- Mayor, wife in spotlight -
As prosecutors investigated the motive, the mayor and his wife, who is the president of the local family services department, came under the spotlight.
The couple, who have apparently gone into hiding, have been accused of having links to the Beltran Leyva drug cartel, which created the Guerreros Unidos, according to El Universal newspaper.
Governor Angel Aguirre said one of the theories being checked by investigators is whether students' presence in the city caused "much discomfort" to the wife, Maria de los Angeles Pineda Villa, because she was giving a speech that day.
When a party started after her speech, she was told that the students were trying to seize buses, Aguirre said. The students say they were in Iguala to raise funds for their studies.
El Universal cited a CISEN intelligence services report as saying that Pineda told her security chief to ask the police to repel the students because she feared they would interrupt her event.
After a clash, the public security director ordered his men to stop the students' buses. When the students got out, the officers began to shoot, killing three, the document says.
While Mayor Jose Luis Abarca Velazquez denied knowing about the attack, the CISEN document says he told the police director to chase and punish the students.
An official in the attorney general's office refused to confirm the document's authenticity, citing secrecy requirements during the investigation.