In their ICJ plea they have raised questions over the case investigation. "Factors of caste and creed, financial status, political approach and pressure have become so important that investigators are hand-picked for particular cases and where the establishment, so wishes, the investigation is tailored to achieve a particular result. It's also depend upon the political power," they said in the petition filed through advocate A.P. Singh.

"The malaise runs deeper. The political class is using investigating agencies as tools for partisan political objectives. Heads of agencies are chosen to do hatchet jobs," it claimed.

Referring to coronavirus and the deteriorating air quality, they said, "Everyone is aware of what is happening in the world and also in Delhi NCR in regard of water and air. Life is going short to short, then why death penalty?"

Earlier an apex court Bench, comprising Justices Arun Mishra and M.R. Shah, heard submissions of lawyer M.L. Sharma, appearing for Mukesh, and rejected the plea by terming it "not maintainable" as the review and the curative pleas in the case had been dismissed.

"Circumstances say there is no remedy left. You (Mukesh Singh) have availed mercy plea. It was rejected. Warrants issued. Curative petition has been dismissed. What's the remedy left," the Supreme Court said.

The four convicts -- Mukesh Singh, Pawan Gupta, Vinay Kumar Sharma and Akshay Kumar -- are scheduled to be executed on March 20 at 5:30 a.m.

The case pertains to the gang-rape and murder of a 23-year-old girl, dubbed later as Nirbhaya, in the national capital in December 2012.

Six people, including the four convicts and a juvenile, were named as accused. Ram Singh, the sixth accused, allegedly committed suicide in Tihar Jail days after the trial begun.

The juvenile was released in 2015 after spending three years in a correctional home.