New Delhi, Aug 7 (NDTV): A death threat has been made in writing to one of the three judges who refused to stop the hanging of 1993 blasts convict Yakub Memon. The Delhi Police has registered a case and are investigating an anonymous letter sent to Justice Dipak Misra.

The short note addressed to Justice Misra allegedly says "irrespective of the protection you may avail, we will eliminate you."

Soon after Yakub Memon was hanged in the early hours of July 30, security for Justice Misra and his two colleagues was increased, based on concerns and a threat perception.

The three judges rejected Memon's eleventh hour appeal to stop his hanging in an unprecedented hearing that took place in the Supreme Court in the middle of the night.

Justices Misra, Amitav Roy and Prafulla Pant spent two hours starting at about 3 am on July 30 weighing the new petition, before they ruled they would not stop his execution hours later.

The judges said that Memon had ample time and opportunity to challenge his death sentence and that he had exhausted all legal options available to a death row prisoner.

Memon was convicted in 2007 as the "driving spirit" of the deadly attack in Mumbai in 1993 in which 257 people were killed as bombs exploded back to back in landmarks across the city including the Stock Exchange. In 2013, the Supreme Court upheld his death sentence. A mercy petition filed by his brother on his behalf was turned down by President Pranab Mukherjee.

In the last three days before he was executed, Memon's lawyers, helped by activists, explored several ways to stop his hanging. They said the Supreme Court had wrongly confirmed his death sentence before he had exhausted all his legal options.

The day before his hanging, Memon filed a mercy petition and after it was rejected by the president, he asked for a 14-day delay. But the judges said stopping his execution would have been "a travesty of justice."