Njiru will be detained for 14 days so as to allow the police to complete investigations on the matter.
The deceased was a prison warder at Murang'a GK prison and questions about her whereabouts started after she failed to show up for work on Tuesday.
Murang'a County Police Commander, Josphat Kinyua on Wednesday stated that Wangari's lifeless body was found at her rental house.
“We have started investigations to establish the cause of the death of the prison warder, although by the look of things, it appears that it could be love gone sour,” Kinyua told journalists.
A report by Citizen Digital cited that CCTV footage from the victim's rented house in Kiharu had provided crucial leads on who the constable was with at the time of her murder.
A middle-aged man with a tattoo on his neck was captured arriving at Wangari's home on Monday at 9pm and left on Tuesday at 3am.
The man was carrying a TV set, the deceased’s large bag and her mobile phone as he left.
"One of Ms Wangari’s friends who watched the footage at DCI offices in Murang'a on Wednesday, says she knows the man as Ms Wangari’s friend," the publication further disclosed.
It is alleged that detectives found a roll of bhang superficially placed between her lips and a kitchen knife clutched in her right hand.