Kolkata police sleuths have arrested three persons, including a doctor, for their alleged involvement in sending a death threat to former West Bengal chief secretary Alapan Bandyopadhyay, a senior officer said on Tuesday.

Bandyopadhyay had recently received a death threat through a letter sent to his wife.

Acting on a tip-off, a typist was arrested on Monday from Ballygunge area, who had admitted that he had written the letter using a typewriter, the officer said.

After getting information from him, Kolkata police sleuths arrested the doctor, who works in a city-based medical college, from Raja Rammohan Sarani and his driver from Manicktala area,he said.

"The doctor had sent his driver with a draft of the letter to the typist. He has written such letters to several people. It seems that he has a mental disorder. We are probing into the matter," the officer said.

Bandyopadhyay, who retired in May, is at present the chief advisor to chief minister Mamata Banerjee.

His wife Sonali Chakrabarty, vice-chancellor of Calcutta University, had received the typed letter. "Madam, your husband will be killed. No body can save the life of your husband," read the letter, dated October 22, a police officer had said.