The anonymous caller, who had threatened jail authorities of "snatching away militant inmates from jails", might have used a Voice over Internet Protocol (VoIP) to make the call, police suspect.
"We got a cell phone number of the caller but we are suspecting that it is not the actual number and the caller might have used VoIP," a police high-up, involved with the investigation of the call, told The Daily Star yesterday.
On September 12, Lalmonirhat's jail superintendent and district commissioner received a call from an anonymous person, who threatened to take away militant inmates from the jails.
Before the threat, the jail super and DC also received a letter inside the post office's yellow envelope. The letter read, "We will soon take away our fellow members, who are now kept in your district jail."
Following the letter and call, the Inspector General (IG) of Prisons Brig Gen AKM Mustafa Kamal Pasha on Sunday instructed all prisons of the country to remain on high alert.
The prison chief also asked to form a special task force, issued an 18-point instruction and directed all to keep a close watch on the movement of all inmates, specially militants, top criminals and others accused in sensitive cases like those of the Border Guard Bangladesh.
According to sources, there are around 20 militant inmates in Lalmoirhat district jail. They are members of banned militant outfit Jamaat-ul-Mujahideen Bangladesh and Ansarullah Bangla Team.
None of the 20 inmates, however, are prominent leaders, according to sources.
A jail source said, as of yesterday, there are around 656 militant inmates in different jails across the country. Leaders and top-listed militants of outfits were being kept in Kashimpur high security jail.
The Counter Terrorism and Transnational Crime (CTTC) unit of Dhaka Metropolitan Police, the specialised unit dealing with militancy issues, have already started their investigation into the threat.
Speaking to The Daily Star yesterday, Saiful Islam, deputy commissioner of the CTTC unit, said that they have already started investigating using the the anonymous caller's phone number as a lead.
Meanwhile, after the threats were made, a state-run intelligence agency already suggested shifting the 20 militant inmates from the Lalmonirhat district jail to a high security jail.
Lalmonirhat DC Md Abu Zafor said that he first received the letter a week ago and then received the phone call on September 12. "We have already filed a General Diary with the local police station and an investigation is underway."
He added that the 20 militant inmates were in the district jails as their trials were still underway and that police are suspecting the calls came from the militant outfits the inmates belong to.
Kishor Kumar Nag, superintendent of Lalmonirhat district jail, said usually they always ensure special security measures for militant inmates and there is no scope for loopholes.
About transferring them to a high security jail, he said that no decision regarding their transfer has yet been taken but security has been beefed up.