Several countries are putting pressure on Mexico to solve the disappearance of 43 missing students who vanished last month after clashing with police in the town of Iguala in the state of Guerrero.
The US and the Organisation of American States (OAS) have joined appeals for the country to find the young men, who were last seen 10 days ago.
"This is a troubling crime that demands a full, transparent investigation," said US State Department spokeswoman Jen Psaki on Wednesday. "The perpetrators must be brought to justice."
Jose Miguel Insulza, OAS Secretary General, 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".
Fears over the fate of those disappeared rose over the weekend after 28 unidentified burnt bodies were found in a mass grave outside Iguala.
Two hitmen from a drug gang told investigators they had executed 17 of the 43 young men and dumped them in pits.
Authorities say it will take at least two weeks for DNA tests on the bodies in the mass grave to confirm the victims' identities.
Guerrero's governor said one theory was that the police attacked the students because the mayor's wife was concerned they would disrupt a speech she was giving that day.
The newspaper El Universal cited a Mexican 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 clashes began, 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.
Federal police deployed
The president of Mexico, Enrique Pena Nieto, disarmed the municipal police force and deployed hundreds of federal police in Iguala on Monday.
Authorities also say the municipal police and its gang allies shot at buses carrying the students on the night of September 26.
The attorney general's office has dispatched 30 investigators, criminologists and forensic experts to investigate the case.
The rest of the police force was taken to a military base in central Mexico on Tuesday to undergo evaluations while investigators check if their guns were used in any crimes.
Authorities have detained about 30 people in the case, including 22 Iguala police officers, while the mayor and city security chief are on the run.
A year and a half ago a leftist activist Arturo Hernandez was shot and killed in Iguala and his body was dumped in a pit.