The prosecutor of Minas Gerais state said in a statement, "This morning, four children and a teacher burned to death when a guard at the nursery school sprayed alcohol on the victims and his own body before setting them alight".
Reports revealed the security guard, identified as Damiao Soares dos Santos, 50, was mentally ill and had worked in the centre since 2008.
There were 80 children in the day care when the incident happened, and the police have confirmed four children and a teacher have died, while nine others are in a critical condition.
About 25 people, mostly children, were injured in the attack and are being treated at local hospitals for burns.
He also set himself on fire, police said.
He was taken to hospital, suffering from burns, and later died.
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The security guard is accused of dousing people inside the day care center with flammable liquid before starting a fire.. The motive of the attack was unclear. Local newspapers report he was recently fired from the nursery.
I'm very sorry about this tragedy involving children in Janaúba.
Brazilian President Michel Temer expressed his condolences via Twitter.
Mr Temer said that, as the father of a school-age child, he understood "this must be an extremely painful loss" for the parents.
He allegedly came to the center Thursday to turn in paperwork before starting the blaze.
Janauba's mayor has declared a seven-day mourning period.