"This then created an explosion ... and the fire quickly spread to the daycare centre.
"At the daycare facility, apparently the roof was painted with a highly flammable chemical. Appearently, it was also used to insulate, and to keep the centre warm during winter.
"So there will be some serius questions about the safety measures taken at the centre."
More than 140 children, aged from six months to five years old, were inside the ABC nursery when the fire broke out.
Authorities said smoke inhalation killed many children before rescuers could reach them.
Firefighters carried injured children through the building's only working exit and through large holes that a civilian knocked into the walls before rescue crews arrived, according to a fire department official.
An inspection on May 26 found that the building, a converted warehouse with a few windows mounted high up, complied with safety standards, according to Daniel Karam, the director of Mexico's Social Security Institute, which outsourced services to the privately run daycare.
Guadalupe Arvizu, who was visiting her injured two-year-old grandson at a hospital, said the building has an emergency exit but it could not be opened on the day of the fire.
"The place is in bad condition. It's a warehouse. There are no windows in the classrooms,'' she said.
Building safety in Mexico has been questioned after previous deadly incidents.
In 2000, a fire killed 21 people at a Mexico City nightclub that only had one available exit, lacked smoke detectors and did not have enough fire extinguishers. The emergency exit had been locked with a chain.
Last year, 12 people died in a police raid at another Mexico City nightclub. Officers blocked the crowded club's lone working exit, creating a deadly stampede.