The 5-year-old girl was stabbed in her abdomen with a paring knife in front of a food stall in Nakhon Chai Si district on Sunday and later pronounced dead at a hospital, police said.
Police spokesman Kissana Phathanacharoen said the attacker, Jitrlada Tantiwanitchasuk, has a history of mental illness and is now under custody.
She turned herself to the police before being sent to a psychiatric ward on Sunday night.
“Investigators will go to the hospital to see whether she is in a condition for charges to be filed against her,” Col. Kissana said. “If not, we will seek the court’s approval to admit her to a mental health institution before we can question her and file charges against her.”