Police on Tuesday laid another charge against alleged serial killer Sararat "Aem" Rangsiwuthaporn after police said they had evidence showing she poisoned a 15th victim with cyanide in 2015.

According to police in Thong Lor -- the area where the death of the 15th alleged victim, Monthathip "Sai" Khao-in, occurred -- on Tuesday invited the doctor who performed the autopsy, for questioning.

The doctor was said to have done everything required in the autopsy process but had not stored Monthathip's blood and tissue samples, police said.

According to the investigation team, Ms Sararat and Monthathip co-owned a shop in Wat Phra Pathom Chedi in Nakhon Pathom in 2015. That same year, Monthathip was invited to join a private savings scheme, with Ms Sararat in debt to her for more than 100,000 baht.

Monthathip was found dead in her apartment in Thong Lor on July 7 of that year. The autopsy showed that heart failure was the cause of death. Many belongings, including the house key that Ms Sararat held, were reported to have been lost after her death.