The warnings were sent to the National Cancer Institute and to Prasat Neurological Institute, a source revealed on Sunday.
“A total of three warning letters were sent to these two institutes before the blast occurred at the hospital,” it added.
The source did reveal the woman’s name but said she was a 51-year-old living in Bangkok’s Prawet area.
“She is now being detained at the 11th Military Circle. She has denied any connection to the letters, the same source said.