Three persons, including the owner of a cyber cafe, were taken into custody on Friday after top government officials received e-mails saying that attempts would be made on the lives of President Kalam and Prime Minister Manmohan Singh.
Several senior officials, including the Home Secretary and DGP received the e-mail sent from a single ID.
The mail, which originated from a Kochi village, contained references to Mohammed Afzal Guru, who is facing execution for the 2001 Parliament attack, and PDP leader Abdul Nasser Madani, who is being held in connection with the Coimbatore blasts.
State Home Minister Kodiyeri Balakrishnan, who hurriedly convened a meeting of top officials, including the DGP, chief secretary and the Home Secretary in Thiruvananthapuram, said there was no reason to panic.
The content of the message was that there was an immediate threat to the life of Prime Minister Manmohan Singh, who is slated to visit the place on November 1, and a similar threat to President APJ Abdul Kalam, state police chief Raman Srivastava said after the meeting.
There is no change in the Prime Minister's visit, but security would be further stepped up. The investigation in the case would be directly supervised by Srivastava and Central intelligence agencies have been informed of the matter.
Though police has some clues, the details, including the e-mail ID from which the message was sent, could not be disclosed as it would affect the investigation, Srivastava said.
The involvement of any particular outfit has not been established but surveillance of organisations like the banned SIMI has been stepped up, officials said.
According to officials, the e-mail was sent to most district police heads and other senior police officials simultaneously.
They said a "serious investigation" had already begun and the help of cyber experts would be sought, if necessary, to track down the source of the message.
The officials said the e-mail could possibly be part of efforts to divert the attention of police and security personnel.
Meanwhile, those taken into custody are being interrogated by a police team.
The Prime Minister is scheduled to arrive in Thiruvananthapuram for the inauguration of the golden jubilee celebrations of the state's formation on November one.