A lawyer has been arrested here for allegedly writing a threat letter to a CBI judge hearing the case of arrested TMC strongman Anubrata Mondal, the police said.

The accused, Sudipta Roy, was nabbed from BNR More in Asansol city on Monday night for allegedly threatening special CBI judge Rajesh Chakraborty with a drug case if Mondal, the TMC Birbhum district president, is not granted bail in a cattle smuggling case.

He was nabbed after his mobile tower location was tracked down while he was attempting to return home, a police officer said.

During investigation, it was revealed that the accused, due to personal enmity with Asansol District Court head clerk and TMC leader Bappaditya Chattopadhyay, had impersonated him, using his name and official stamp, while sending the threat letter, the officer said on Tuesday.

"The accused had sent the letter to the CBI court judge on August 23 by speed post to frame Chattopadhyay, his old enemy," he said.

The arrested lawyer had allegedly threatened Chattopadhyay of dire consequences a few days ago, he added.

Chattopadhyay, in a secret statement under Section 164, had on August 26 stated about Roy's threat.

Following this, an investigation was initiated by the Asansol-Durgapur Police Intelligence Department.

Last week, Chakraborty had written to Paschim Bardhaman district judge, urging him to take note of the threat and bring it to the notice of the registrar, judicial service, and appellate side of the Calcutta High Court.

A fake Aadhaar card in Chattopadhyay's name was found in the possession of Roy.

During investigation, it was found that Roy had earlier forged a stamp of a judge and was also booked in a drug case, the police officer said.

Mondal was arrested by the CBI on August 11 in a cattle smuggling case. He is presently in judicial custody in Asansol.

The special CBI court in Asansol is hearing coal and cattle smuggling cases.