New Mexican President Andrés Manuel López Obrador has signed a decree to open a truth commission to investigate the disappearance of 43 students.
The student teachers were kidnapped from the town of Iguala in 2014, and subsequently killed, allegedly by members of the security forces.
The announcement was made on Mr López Obrador's first working day in office.
There have been no high-profile prosecutions for the murders, which shocked Mexico and the world.
The 65-year-old, popularly known by his initials Amlo, won a landslide victory in July - his third presidential bid. He was sworn in as president on Saturday.
Opening an investigation into the Iguala killings was a key election promise for the new president. Despite the outrage over the case, correspondents say the lack of meaningful action or any prosecution of those involved was a stain on the previous administration.
"The whole government is going to help with this plan", he told the victims' families who had gathered for the event, "and I can assure you that there will be no impunity either in this sad and painful case or in any other."
In his first early morning news conference on Monday, which started at 07:00 local time, he promised to bring "deep and radical" change to the country. The event marked a changed from the behaviour of his predecessors, who rarely held conferences.
"Isn't that a change, that I am here, informing you?" he asked reporters.
The president has also started to press ahead with other campaign promises, such as selling the presidential plane as part of a drive to reduce government luxuries.
On Sunday, he took his first flight as president on a commercial aircraft with the rest of the passengers.