Thai police have widened their investigation into a woman accused of a spate of cyanide poisoning murders, with officers on Thursday raising the number of victims to 13 and charging her with premeditated murder.

Sararat Rangsiwuthaporn, who is married to a senior police officer, was arrested on Tuesday over nine alleged murders which took place over several years. Authorities didn't identify all of the alleged victims, but named Sararat's former partner, as well as two female police officers, among the dead, BBC News reported.

Police believe money was the motive in the killings but said that Sararat — who is four months pregnant — has previously been diagnosed with psychiatric issues.

Officers were now investigating at least 13 suspicious deaths dating back to 2020, deputy national police chief Surachate Hakparn said Thursday.

"She has been charged with premeditated murder," he told reporters in Bangkok.

Police have not specified how many murders Sararat has been charged with, but they say she denies all the allegations against her. On Tuesday, police said they collected fingerprints and other evidence from Sararat's Toyota Forerunner.

Police have also expanded the geographic area they are investigating to five provinces, most to the west of Bangkok.

Officers found a substance at the woman's home that authorities believe to be cyanide, and suspect she poisoned the victims' food and drink.

Following routine health checks in prison, the Department of Corrections confirmed Thursday that Sararat is four months pregnant and experiencing stress, blurry eyes and headaches.

Investigators have interviewed her police officer husband and other witnesses.

Police described how a fourteenth person narrowly escaped death after vomiting up poisoned food.

"The suspect lured her latest victim into eating a herb, and around 20 minutes later she collapsed," Surachate said.

He urged the public to contact police with any information about other potential cases.

Police initially suspected the woman of murdering a friend in Ratchaburi province, west of Bangkok, about two weeks ago.

Local media said the victim collapsed on the bank of the Mae Klong River after releasing fish as part of a Buddhist ritual. BBC News reported traces of cyanide were found in Siriporn Khanwong's body during the autopsy, police said. Her phone, money and bags were also missing when she was found.

After questioning the suspect, investigators linked her to other cyanide poisoning cases.

Cyanide can be detected in corpses several months after death, if a lethal amount was used, BBC News reports. The poison starves the body's cells of oxygen, which can induce heart attacks. Symptoms include chest pain, dizziness, shortness of breath and vomiting, the CDC says.

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• Thailand
• Murder