Two years ago Monday, 43 students left their teachers' college in Mexico to attend a protest and never returned. The Iguala mass kidnapping, which occurred in the country's Guerrero state, remains a tragic topic for families of the missing men, who are all presumed dead, and a sore one for the Mexican government, which has been accused of mishandling the case.

"Since Sept. 26 [2014] our lives have stopped," Mario César González Contreras, the father of one of the missing students, told NBC News last month. "My wife and I left our jobs to move to the school. And we are still there searching for our son and his classmates."

On the anniversary of the Ayotzinapa Rural Teachers' College students' vanishing, read on for the basic facts and developments that have occurred since the mass killings:

What happened? The students left their Ayotzinapa school to find buses they could take over and use to get to a demonstration in Mexico City. But police and soldiers stopped the buses and started shooting. The students fought back with rocks, but when the conflict was over dozens of them were injured or missing. By Sept. 30, 2014, many had been found, but 43 people couldn't be accounted for, according to the Intercept.

Were any of the missing students found? Just one set of remains has been conclusively identified.

What was the government's reaction? Officials under President Enrique Peña Nieto launched an investigation, arresting and questioning dozens of people as huge groups of protesters blocked the streets of Mexico City to call for transparency.  In a news conference that November, Mexican Attorney General Jesus Murillo Karam blamed the mayor of Iguala, José Luis Abarca, for asking police to stop the group so it didn't disrupt an event his wife was holding nearby.

Murillo Karam alleged that the police gave the 43 students to a local drug gang, Guerreros Unidos, which killed them and burned the bodies, VICE News reported.

So... case closed? Nope. Within weeks, experts were releasing reports and families were giving interviews saying they doubted the results of the government investigation. Earlier this year, court documents showed witnesses accused investigators of torturing them so their stories aligned with the official account, the Associated Press reported. Evidence may have been planted, as well.

The probe was reopened, and an independent panel came to its own conclusions — despite what Reuters called "the government's stonewalling." The independent analysts said the government's theory was basically right but should have involved an investigation looking into federal police, as well.

"There seems to be no limit to the Mexican government's utter determination to sweep the Ayotzinapa tragedy under the carpet," Amnesty International's Erika Guevara-Rosa said in a statement at the time.

Murillo Karam stepped down in February 2015.

How is Peña Nieto involved? The Mexican president has been widely criticized for the way he handled the case, and his approval ratings have taken a hit. The victims' families accused Peña Nieto of covering up the gang violence the night the students vanished in hopes of saving his political reputation, Time reported. Peña Nieto reportedly didn't visit the scene of the crime for months and instead promoted an economic package, according to Al Jazeera.

"We're on the same side," he insisted last year. "You and I are seeking the same thing: to know what happened to each and every one of your sons."

But many relatives have remained convinced the government's corruption is overshadowing the Iguala kidnapping case.

What's happening now? Just last week, Eber Betanzosa, the deputy attorney general for human rights, told Agence France-Presse the Mexican government still had an open investigation into the incident that "won't rest until this is cleared up." While meeting with the students' families Wednesday, a United Nations human rights representative applauded the decision to keep analyzing the case.

The families scheduled rallies for Sunday, Monday and Tuesday in Mexico, Cinequo reported. They've recently ramped up their demonstrations aimed at forcing Peña Nieto to resign.