The attack is one of the worst mass slayings since President Andres Manuel Lopez Obrador took office 19 months ago pledging to reduce record levels of violence.
The killing of 26 people in an unregistered drug rehabilitation center in central Mexico is the deadliest such attack in a decade and has led to calls for change in a prosperous state that has become a cartel battleground.
Ricardo Ortiz, mayor of the city of Irapuato in Guanajuato state, raised the death toll from 24 Thursday after two of the seven people injured in the attack died. Ortiz said the other five are out of danger.
Ortiz said there were six or seven attackers; they pulled up in two vehicles at the modest two-story house on the outskirts of Irapuato.
Photos from the scene shared by police with local reporters showed at least 11 prone and bloodied bodies lying in a room.
The attack was one of the worst mass slayings since President Andres Manuel Lopez Obrador took office 19 months ago pledging to reduce record levels of violence.
But homicides hit a new record last year and are trending higher still in 2020.
On June 6, 10 men were killed when gunmen opened fire in a rehabilitation centre for addicts in Irapuato. It was not immediately clear if the same facility was hit both times.
Guanajuato, a major carmaking hub, has become one of the principal flashpoints of criminal violence in Mexico.
Rehab centres are known to have been targeted by criminal gangs waging turf wars for control of the drug business.
At least 26 people were killed in an arson attack by suspected gang members on a bar in the southern Mexican port of Coatzacoalcos last August.