A massive march wended its way through the center of Mexico City on Wednesday evening in the latest expression of public outcry demanding justice for 43 teaching students of the Ayotzinapa Normal School in Guerrero who disappeared in September after an encounter with police.

Tens of thousands of people streamed back into the Zocalo main square in the city's historic downtown two weeks after a similarly huge nighttime demonstration took place there. The protesters came from a variety of backgrounds and professions, and included prominent activists of previous movements calling for peace in Mexico, such as poet Javier Sicilia.

Six weeks after the September 26 attack that left six dead, more than 20 injured, and the 43 normalistas missing, the movement that has taken to the streets sticks to a core message: "They were taken alive — we want them returned alive." Parents and surviving Ayotzinapa normalista students told news outlets that they expected the government to soon tell them that the students are dead, adding that they would wait for the findings from a special team of Argentine forensic experts before abandoning hope.

On Tuesday, authorities announced the arrests of Jose Luis Abarca, the fugitive mayor of the town where the students disappeared, and his wife and alleged cartel co-conspirator, Maria de los Angeles Pineda. To help readers who might be unfamiliar with this story, here is a timeline of key events to understand the ongoing case of the missing students.

September 26: A group of roughly 120 students from the Ayotzinapa Normal School near Tixtla, Guerrero, enter the city of Iguala to protest education reforms and raise money to attend an upcoming demonstration in Mexico City. They eventually commandeer, or "borrow," three coaches from a bus station. Mayor Jose Luis Abarca allegedly orders police stop and detain the students after they attempt to disrupt a public reception being held that evening to bolster his wife's political ambitions.

At about 8:30 PM, Iguala municipal police and other armed men surround and ambush the three buses carrying the students. As dozens of them disperse and escape into neighboring streets, dozens of others are grabbed and loaded onto police vehicles.

At about midnight, as other officials and news reporters gather at the scene, another convoy of armed men begin firing at the buses. Two students are killed that night, as well as three bystanders: a bus driver, a woman in a taxi, and a 15-year-old soccer player.

September 27: The body of a fourth student, the sixth confirmed victim of the attack, is discovered. Julio Cesar Mondragon, a 22-year-old father from Mexico City, is found with his facial skin and eyes removed — a cartel-style execution. Survivors of the attacks and others attempt to locate the students who were taken away at jails and police stations, but they are nowhere to be found.

The 43 normalistas from Ayotzinapa, all young men early into their college careers, are declared missing by their classmates and parents.

September 28: Authorities arrest 22 Iguala municipal police officers for their involvement in the attacks, allegedly carried out with the Guerreros Unidos cartel. The officers' weapons show signs of being recently used, and 19 of the officers test positive for gunpowder residue.

September 29: In a radio interview, Abarca declares that he has no information about the case and denies that he ordered police to attack the buses. He says he had initially heard that masked young men were disturbing the peace in downtown Iguala. "These young men always provoke the authorities," he says.

"I was dancing," he notoriously remarks elsewhere.

Mexican President Enrique Peña Nieto cancels a scheduled trip to Guerrero state, in the first signal of a federal-level response to the severity of the incident.

Survivors describe police attack in Mexico: 'If you moved, they fired. If you yelled, they fired.' Read more here.

September 30: Mayor Abarca requests a 30-day leave of absence, theoretically to avoid compromising the investigation into the police attack. During his statements, he says that he will support every effort to locate and prosecute those responsible, "whomever they may be."

October 1: Governor Angel Aguirre orders Jose Luis Abarca to "present" himself to authorities, but he is nowhere to be found.

October 4: Forensics teams locate four mass graves that could potentially hold the remains of the missing students. Authorities initially declare that 28 bodies are found there.

October 6: In a VICE News report, Ayotzinapa students describe the attack of September 26 and explain how customary the practice of hijacking commercial buses has become for them, as normal schools are notoriously neglected by the Mexican education ministry.

Two guerrilla groups active in Guerrero release statements on the Iguala attack, calling for an "offensive" against the Mexican state.

For the first time, Peña Nieto directly addresses the Ayotzinapa crisis in a national address. "Mexican society, and the families of the young students who are sadly missing, rightly demand clarification of the facts and that justice is done," he says.

October 8: The first large-scale demonstration against the Iguala attacks and the students' disappearances occurs in Mexico City. Ayotzinapa students lead the march.

October 10: The United Nations Office of the High Commissioner for Human Rights releases a statement calling the Iguala attacks a "crucial test" for Mexico's government as it confronts mounting evidence of human-rights failures and abuses in the aftermath of the September 26 shootings.

"What happened in Guerrero is absolutely reprehensible and unacceptable," the statement says. "It is not tolerable that these kind of events happen, and even less so in a state respectful of the rule of law."

October 11: The state government announces that some of the bodies discovered in the first set of mass graves do not belong to the missing students, alarming the public that clandestine burial sites linked to drug-war violence can be found throughout Guerrero.

October 12: A survivor of the police attack tells VICE News that the armed men who shot upon the students "looked like state police, because of how they were equipped, and they told us, 'Sons of bitches, you're getting the fuck out of here! Get on your buses and get the hell out, you're not welcome in this city!' "

October 13: Dissident teachers, normalistas, and other masked individuals storm the statehouse in Guerrero's capital, Chilpancingo, holding hundreds of state employees and civilians hostage inside the building for hours. Shortly after releasing those inside the complex, the protesters set the facade of the building on fire using Molotov cocktails.

Demonstrators also take over a bread truck, flipping it over, before setting it on fire.

October 14: A presumed leader of the Guerreros Unidos drug gang allegedly commits suicide during a federal operation aimed at capturing him in the neighboring state of Morelos, the attorney general's office says in a statement. Benjamín Mondragón is said to have taken his own life during the shootout.

"He looked out [a window], simply made an expression, and then shot himself in the head," one official says.

October 16: Peña Nieto again addresses the Ayotzinapa case in public. He says that solving the case is a "priority" of the Mexican state. Students at colleges and universities across Mexico City and the country declare a two-day strike in support of Ayotzinapa.

October 17: Mexican Attorney General Jesus Murrillo Karam announces the arrest of Sidronio Casarrubias Salgado, the alleged leader of the Guerreros Unidos cartel. Casarrubias reportedly tells authorities that the confrontation with the Ayotzinapa Normal School students was "a casual situation."

October 19: In a VICE News report, a volunteer community police force describes lackluster search efforts on the part of government officials, as volunteers comb the hillsides around Iguala for signs of the missing.

Federal officials also announce the takeover of 13 municipalities in Guerrero and neighboring states, relieving local police forces of responsibility for public security.

October 22: Thousands of people take to the streets again in Mexico City and in major cities worldwide in response to the Iguala police attacks.

In Iguala, Ayotzinapa students return to the scene of the attacks for the first time, along with parents of the missing and various supporters. After occupying teachers leave, other masked individuals arrive and loot Iguala city hall. These attackers also later move to ransack a mall that is linked to the fugitive mayor.

In Mexico City, Attorney General Murrillo Karam confirms that Mayor Abarca and his wife directly ordered the attack against the students.

October 23: The governor of Guerrero, Ángel Aguirre, resigns under growing pressure from demonstrations in his state and around Mexico. "Thank you to all of Guerrerenses who accompanied me, those who gave me their confidence, and support," Aguirre tweets.

October 29: Dozens of parents and family members pile into buses to travel from Ayotzinapa to the presidential residence Los Pinos in Mexico City to meet President Enrique Peña Nieto. The meeting lasts five hours and ends inconclusively for the parents, who declare that they remain frustrated and disappointed after the meeting.

November 4: Fugitive Mayor Jose Luis Abarca and Maria de los Angeles de Pineda are arrested in a run-down house in a poor barrio in the Iztapalapa borough of Mexico City.

Authorities say little about the arrest in a subsequent press conference, but a video that is later made public shows Abarca and his wife being shuffled out the house by federal police officers at around 2:30 am that morning. "Don't touch me, who do you think you are?" Pineda reportedly snaps at an officer who attempts to lead her away by the arm.

November 5: Another massive demonstration for the Ayotzinapa students takes place on Paseo de la Reforma and on the Zocalo main square in Mexico City. Scores of demonstrators call for the resignation of Peña Nieto and repeat the call for the return of the missing students.

November 6: In a press conference in Mexico City, top representatives of Human Rights Watch call the Ayotzinapa case the worst human-rights crisis facing Mexico since the 1968 massacre of unarmed students at Tlatelolco.

"Impunity in Mexico is the rule, not the exception," says Jose Miguel Vivanco, director of the organization's Americas division.