"Three persons, including Narendra Prasad Singh, whose cell phone might have been used for sending the SMS have been detained," Senior Superintendent of Police Vinit Vinayak said.
"We are probing it from all possible angles for taking to a logical conclusion," Vinayak said referring to the threat, purportedly issued by the banned CPI (Maoist), to kill Kumar if two of their top leaders were not released from jail.
'Dabloo', a Naxalite leader, in an SMS from the mobile phone number - 09308670993 - had claimed to a private TV channel reporter that there was a plan to kill the Chief Minister. The message demanded release of two Naxalite leaders 'Raviji' and Diren Singh.
Following the threat, the Chief Minister's security had been tightened, state Home department Principal Secretary Amir Subhani said.
When contacted, Kumar said that he was least bothered about such threats.