The murder, in which the victim had her throat slashed, took place on Monday night (July 9).
Siwaporn Jaisook, 31, was arrested in her room on the third floor of an apartment building located 100 metres away from the victim’s hair salon, while the teenager, who is five months pregnant, was apprehended at a shack nearby.
The arrests came after CCTV footage captured them leaving the scene of the alleged murder and heading toward the apartment building.
Police also seized B6,000 in cash, a gold bracelet weighing one baht, a black purse and bloodstained clothing from the suspects.
Both confessed during questioning that they killed Wannapat Chakhamnan and fled with her valuables, investigators said.
Siwaporn said she and her young accomplice had known each other for nearly a year and met while working for a firm making electrical fans in the district.
They later became unemployed, ran out of money and hatched a robbery plan, police said.
On the day of the killing, both went to the hair salon posing as customers, according to police.
The teenager then allegedly acted as lookout while Siwaporn dragged Ms Wannapat to the back of the salon and demanded the victim hand over cash and valuables at knife-point.
However, the woman resisted prompting Siwaporn to slash her throat, police said.
Siwaporn claimed she committed the crime partly because she felt sympathy for the girl who needed money to take care of herself during her pregnancy.
Phra Samut Chedi police said they were called to the hair salon, named Porn Beauty-Salon, on Soi Laen Phra Mery-Wat Khu Sang in tambon Nai Klong Bang Pla Kot around 1am yesterday.
Upon arrival, officers found the victim dead on the floor at the back of her hair salon with her throat slashed.
A bloodstained knife was found on top of a washing machine next to the body.
Police also found the victim’s yellow purse on her bed above the shop.
Three bloody footprints were found near the bed.
Thanakrit Innangthaen, a 19-year-old neighbour who found the body, said Ms Wannapat had lived alone at the salon.
Her husband and children would visit her once a week, while friends would drop by sometimes.
He said his mother owned a grocery store two shopfronts away from the salon which normally closed at 7pm.
However, on Monday he noticed the salon was still open at 11pm so went in and found the victim lying face down in a pool of blood.
Not realising she was dead, Mr Thanakrit said he called rescue workers who upon arrival discovered her throat had been slit.
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