Kolkata Police have arrested three persons, including a doctor, in connection with a letter threatening to kill former chief secretary Alapan Bandyopadhyay.

Sources in Lal Bazar said that Arindam Sen, the doctor, has been sending threat letters to several high-profile people for the last two years.

The letter issued in the name of Alapan’s wife Sonali Chakraborty Bandyopadhyay came to her on October 26, signed by Gaur Hari Mishra — lab technician of Rajabazar Science College, the science division of Calcutta University.

It also bore the name of Mahua Ghosh — the head of the department of Chemical Technology of the university. Sonali Chakraborty Bandyopadhyay is the vice-chancellor of Calcutta University.

While interrogating Ghosh and Mishra, police realised that they were nowhere involved in the case.

On Monday evening, working on a tip-off, police arrested a typist identified as Vijay Kumar Kayal from Bijan Setu area of Ballygunge in South Kolkata and learnt about Arindam Sen who sent his driver to get the letter typed from him.

According to police, the letter written in English was a one-liner that read: “Your husband will be killed. No one will be able to save your husband.”

In the evening, police arrested Arindam Sen, a resident of Raja Rammohun Roy Sarani and his driver Ramesh Sauke.

Sen, a doctor by profession at KPC Medical College in Jadavpur, has the habit of sending such letters to several high-profile people for the last two years, police said.

Ramesh Sauke used to get the letters typed by Vijay Kumar and posted them from different post offices. The addresses were given by Arindam Sen.

On October 25, Sen made Ramesh Sauke post six letters, including that of Alapan Bandyopadhyay, from a post office at Sarat Bose Road in South Kolkata.

Apart from Sonali, threatening letters were also sent to Principal of NRS Shaibal Mukherjee, Principal of Calcutta Medical College Hospital Julie Bhattacharya, and Director of Medical Education Debashis Bhattacharya.

According to a police source, during interrogation Arindam Sen confessed to his habit of sending threat letters and that he did this to get relief from mental stress.

Since no one had ever filed a complaint against him, it had become his habit, he said, adding that he had no idea that this could lead him to trouble. Sen also confessed that Gaur Hari Mishra was his neighbour and he had no idea about this.

The three arrested will be taken to court on Tuesday. The investigating officers will try to extract more details about Sen, his associates and other recipients of such letters.