Panya Khamrab, 34, a former police corporal at Na Wang Police Station, went on a gun and knife rampage at a nursery in Uthai Sawan Subdistrict on Thursday afternoon before killing his family and himself.
On Friday, the death toll from the massacre rose to 38. Twenty-four of the victims are children, including the killer's son.
Police expect investigations to show that the killer was stressed and intoxicated with drugs, National Police chief Damrongsak Kittipraphat said.
He added that the force would respond by making efforts to tackle problems of drug addiction within its ranks.
"The police will take lessons learned from this incident, including tactics to tackle such incidents effectively and efforts to solve drug addiction issues," he said.
The killer went on the rampage shortly after appearing in court on a drugs charge on Thursday morning, Damrongsak said.
The court was scheduled to issue its verdict in the case on Friday.
Panya had been dismissed from the force on June 15 for possessing methamphetamine.
His mother told police that after Thursday's court appearance, her son had taken drugs before grabbing a gun and driving to the nursery.
The police chief said a blood test has been ordered on the killer’s body.
Meanwhile, Prime Minister Prayut Chan-o-cha has ordered police to set up a command centre and liaise with related agencies to relieve the suffering of victims’ relatives.