Six people were killed last week and more than 40 are considered missing after municipal police officers and other armed men in southern Mexico allegedly opened fire on buses full of students from a rural teacher's college and a third-division soccer club, authorities and witnesses told VICE News.

Three attacks Friday and Saturday near the small city of Iguala in a mountainous region of Guerrero led President Enrique Peña Nieto to cancel a scheduled trip to the state as authorities sent Mexican soldiers and federal police forces to patrol the city where the shootings took place.

State authorities arrested 22 members of the Iguala police force Sunday and kept them behind bars Tuesday in Acapulco. Prosecutors said they would face homicide charges. Testing confirmed the presence of gunpowder residue on sixteen of the arrested officers, officials said, indicating they had recently fired their weapons.

Students from the teachers college, the Raúl Isidro Burgos Ayotzinapa Normal School, came under fire Friday night as they traveled back to the town of Ayotzinapa after soliciting donations in Iguala for supplies for their school, part of a chronically neglected, revolutionary-era teacher training system in Mexico.

The students were also reportedly raising money for a planned trip to Mexico City to participate in an annual demonstration commemorating the Tlatelolco student massacre of October 2, 1968.

The "normalistas," as they are called in Mexico, had asked drivers of commercial bus lines to transport them back to their school, according to students interviewed by VICE News. The police and other armed men allegedly surrounded and confronted the buses on the outskirts of Iguala, under the belief that the students had kidnapped the drivers and buses, they said.

Teachers demonstrated on Monday at the congressional building in Chilpancingo, the capital of Guerrero. (Photo by Pedro Pardo)

It remains unclear why the police and armed men opened fire. Witnesses and reports said the first attack occurred at about 9pm, leaving one student critically injured.

"We thought that they were going to kill all of us, they were hunting us," David Flores, a survivor of the attack and student at the Ayotzinapa Normal School, told VICE News. "I thought that we were done for. It was a miracle that we survived."

After the initial gunfire, dozens of students fled the scene or were detained. As of Tuesday, 43 of these students remain missing, students and human rights activists in Guerrero said.

Three hours after the initial attack, as students who didn't flee sat and waited for authorities to arrive to investigate the crime scene, another armed group showed up and opened fire, killing two more normalistas just after midnight. They were identified as Daniel Solís Gallardo, 25, and Julio César Ramírez Nava, 23. Aldo Gutiérrez Solano, 19, was left brain dead after receiving a bullet in the head.

Around the same time, police or armed men allegedly opened fire on another bus nearby, apparently mistaking it for one carrying normalistas, although its passengers were mostly members of the Avispones, or the Wasps, a third-division football club from the capital city of Chilpancingo.

Three people were killed in the bus attack: the driver, identified as Víctor Manuel Lugo Ortiz, a woman named Blanca Montiel Sánchez, who was in a cab on the same street and hit by a stray bullet, and David Josué García Evangelista, a 15-year-old player for the Wasps.

"They attacked us from the right, the left, and in front," one of the survivors in the soccer club bus attack told El Universal. "After they left we spent a minute in there all screaming. I broke a window and went to hide about 200 meters away."

One of the officers arrested in connection the normal school attacks arrives under arrest in Acapulco for processing. (Photo by Pedro Pardo)

Students said the missing normalistas may have fled into the countryside around Iguala, fearing for their lives. Witnesses said at least 20 of these students were detained by police, although they are nowhere to be found, suggesting they may have been kidnapped.

On Saturday, the body of one missing student was found gruesomely mutilated. Julio César Mondragón Fuentes, a first-year normalista, was found a few blocks away from where the initial attack occurred with his face removed, according to statements by his classmates and graphic photos that circulated on social media.

The casket carrying 15-year-old David Josué García Evangelista, who died from bullet wounds after one of the normal school attacks, on Sunday during his funeral. (Photo by Pedro Pardo)

Guerrero is the second-poorest state in Mexico after Chiapas, and it has a long, sad history of political disappearances and attacks against leftists and indigenous guerrilla leaders, particularly during Mexico's so-called Dirty War in the late 1960s and early '70s. Guerrero also has one of the highest homicide rates in Mexico, and has been the scene of clashes between local militias — known as _autodefensas — _drug traffickers, and police.

The mayor of Iguala, José Luis Abarca, who has been accused of links to armed criminal gangs, said in a radio interview Monday that he had not heard about the attacks.

"I was at the city's social-center dance with my wife and didn't hear about anything," Abarca said. "I didn't hear about any violent acts, I was dancing."

Abarca, a member of the Party of the Democratic Revolution, or PRD in Spanish, asked Tuesday for a temporary, 30-day reprieve from his post as investigations continued.

Iguala, located between Mexico City, the Pacific coast, and western Mexico, is a strategic transit point for both commercial and illicit goods, including drugs. The Mexican newsmagazine Proceso has reported that Abarca's brother-in-law is an alleged lieutenant in Guerreros Unidos, a local cartel, and is also being investigated by federal authorities for links to the Beltran Leyva organization, a larger cartel with a presence in Guerrero.

Soldiers set up a checkpoint at the entrance to Iguala to inspect vehicles. Photo by Pedro Pardo.

Juan Angulo, director of the Guerrero newspaper El Sur, told VICE News that Guerreros Unidos kidnaps people from Cuernavaca, a large city near Iguala, and dumps their bodies in a local river.

"People are kidnapped while riding commercial buses," Angulo said. "This has led civil organizations to state that they believe that the mayor is not very far from this organized crime group, that he tolerates it, or is directly involved."

In a separate case last week, seven military servicemen were arrested in connection to a massacre of 22 people in the neighboring state of Mexico. If tried and convicted, the soldiers would be responsible for one of the worst extrajudicial military massacres in modern Mexican history.

The weekend attacks were not the first time normalistas have come under fire from government security forces. In December 2011, two students from the Ayotzinapa Normal School were killed during a police operation following a blockade of the main north-south highway in Guerrero, during protests in which the normalistas demanded a higher budget for their school. Two state police officers were detained in connection with the killings, but were later exonerated.

In the capital of Chilpancingo on Monday, normalistas protested outside the state congress and expressed outrage at the murders and disappearances. Demonstrators launched rocks, breaking most of the building's windows.