MEXICO CITY (Reuters) - Mexican President Andres Manuel Lopez Obrador set up a commission on Monday tasked with determining what happened to 43 missing and presumed killed students, which could investigate soldiers over the events in 2014 that still haunt the country.

Mexico's new President Andres Manuel Lopez Obrador holds a news conference at National Palace in Mexico City, Mexico December 3, 2018. REUTERS/Edgard Garrido

“This is a matter of state,” Lopez Obrador said at an event with parents of the missing on his first weekday in office, following his inauguration on Saturday. He offered protection to witnesses so they could safely tell their stories.

“We are not going to wash our hands of this,” he said, promising that no obstacle would prevent the truth from being revealed, and enlisting the United Nations to advise on the probe.

The abduction and suspected massacre of the 43 trainee teachers in the southwestern city of Iguala precipitated one of the worst crises of former President Enrique Pena Nieto’s government, as criticism swirled around its investigation into the case.

Monday’s announcement was attended by parents who wore shirts emblazoned with the faces of their missing children. Family members counted out until they reached “43,” and at times shouted to the president, “Don’t fail us!”

For the first time, the army could be investigated by the commission, Lopez Obrador said, a key demand of family members and experts who believe that members of the military have information about the case that remains secret.

“The investigation has to include all of the government, anyone involved,” said Lopez Obrador, including any “members of the military that might have been involved.”

Some supporters of the army have said it should not be subject to the same transparency as other parts of the government.

Lopez Obrador’s foreign ministry has also invited international organizations to “assist and cooperate” with the commission, including the United Nations and the Inter-American Commission on Human Rights (IACHR), which will be responsible for managing a group of independent investigators.

“This case need to be open to all investigations. There have been omissions and a lot of sloppy work,” said Angela Buitrago, a member of a group of experts named by the IACHR that said there were flaws in an earlier investigation.

According to the previous government’s assessment, the students from the all-boys teachers college in Ayotzinapa in the violent southwestern state of Guerrero were rounded up by police who handed them over to a gang that murdered them, for reasons that are unclear.

But in a report released in 2015, international experts flagged problems in the official investigation and rejected its claim that the victims were incinerated in a garbage dump.

Lopez Obrador has placed an early focus on trying to pacify Mexico, which has suffered some 200,000 murders and numerous rights violations in a war on drug cartels that began a dozen years ago.