Supreme Court judge Dipak Mishra who handled Yakub Memon's case has received a threat letter at his residence.
Subsequently, security has been beefed up outside the judge's residence.
In the hand-written letter, the senders warned the judge that he would not be spared as "we have already arrived in the city".
Vijay Singh, Deputy Commissioner of Police (New Delhi), said the letter was detected by a security officer near the gates of the judge's residence on Wednesday night.
A case under IPC 506 (criminal intimidation) and IPC 507 (criminal intimidation by anonymous communication) has been registered at Tughlaq Road police station on the statement of a security officer.
The matter is being investigated even as there was no immediate breakthrough, said police.
Security beefed up outside Justice Mishra's residence. Photo: Sandeep Saxena