Siriporn Khanwong died after taking a trip with Rangsiwuthaporn to a region west of Bangkok, where they took part in a Buddhist ritual at a river.
Khanwong collapsed and died on the riverbank soon after.
Police found traces of cyanide in her body.
Investigators now believe Rangsiwuthaporn killed 11 others since 2020, including an ex-boyfriend and two women police officers.
She is believed to have known them all. Police said they died in similar ways to Khanwong.
Cyanide interferes with the body’s ability to use oxygen, and can trigger heart attacks.
Police allege Rangsiwuthaporn may have killed them for financial reasons.
They have also questioned a senior police officer who was Rangsiwuthaporn’s partner until they recently split up.
Rangsiwuthaporn has denied all the charges.
