Supreme Court Justice Dipak Misra, who headed the three-member bench that gave the final judgment on Yakub Memon, received a threatening note which claimed that "irrespective of the protection you may avail, we will eliminate you".

The anonymous note addressed to Justice Misra was found by security guards in the backyard of his home in Delhi, and investigators believe that it was thrown over the wall, which is not guarded by CCTV cameras or security personnel.

The fact that the perpetrators went unnoticed has further given the police reason that the letter is not merely a prank, and those who wrote it might have done a recce of Justice Misra's residence in Tuhglaq Road in the national capital.

According to police, the letter reads that "we have already arrived in the city" and the judge would not be spared.

"The matter cannot be taken lightly. The way with which the letter was dropped shows that the senders are well aware of the area. They seem to know where the CCTV cameras were and the exact time when guards go for a break," said a senior police officer.

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 basis of the police officer's statement.

Investigators told dna that police are continuously keeping a watch of the areas adjoining Justice Misra's residence and other areas where he travels to.

The Supreme Court had rejected Yakub Memon's last-minute plea just two hours before the 1993 Mumbai serial blasts convict was scheduled to be executed in the Nagpur central jail.