The Central Bureau of Investigation has moved the Calcutta High Court, seeking a death sentence for Sanjoy Roy, who was convicted of the rape and murder of a doctor at Kolkata’s RG Kar Medical College and Hospital, The Indian Express reported on Thursday.

The agency described it as “rarest of the rare” case, the newspaper quoted unidentified officials as saying.

The Sealdah Civil and Criminal Court in the city had on Saturday found Roy guilty of the crime. On Monday, Additional District and Sessions Judge Anirban Das sentenced him to life imprisonment.

Das refused to grant the death penalty to Roy, sought by the Central Bureau of Investigation. The court said that the crime did not meet the “rarest of rare” criteria for capital punishment.

The “rarest of rare” doctrine is used in India to decide when to impose the death penalty in cases that are particularly brutal, heinous or depraved. The doctrine is often applied in cases of murder, rape and other serious crimes.

The Central Bureau of Investigation’s decision to challenge the trial court’s sentence came hours after it opposed a plea filed by the West Bengal government in the High Court on Tuesday seeking the death penalty for Roy, The Indian Express reported.

The central agency had argued that only the victim’s family, the investigating authority or the convict can file an appeal. The state government contended that since the incident took place in a state-run hospital, and law and order is a state matter, it should be allowed to appeal.

The bench of Justice Debangshu Basak and Justice Md Shabbar Rashidi said that the court would hear the victim’s family and the convict before deciding whether to allow the state to seek the death penalty.

The 31-year-old trainee doctor was found dead on the premises of the hospital on August 9. The incident had sparked protests across the country.

On Monday, Chief Minister Mamata Banerjee expressed dissatisfaction with the sentence awarded to Roy. “Case was forcibly taken from Kolkata Police, had it been with them, it would have ensured death sentence,” she said.

The case was initially investigated by the West Bengal Police and later handed over to the Central Bureau of Investigation by the High Court.

Banerjee said that she was shocked to see that the trial court did not find that the case met the rarest of rare doctrine.

“I am convinced that it is indeed a rarest of rare case which demands capital punishment,” the chief minister said on social media. “How could the judgement come to the conclusion that it is not a rarest of rare case?!”

After the incident came to light in August, the West Bengal Junior Doctors’ Front led protests for several weeks demanding the resignation of senior officials and an end to the “threat culture” in the state’s medical centres.

Roy was arrested by the police on August 10, the day after the body of the doctor was discovered in the hospital’s seminar room.

Three days later, the Calcutta High Court ordered the Central Bureau of Investigation to probe the matter.

The Sealdah Civil and Criminal Court held a daily trial after the chargesheet was filed in the matter on October 7. Roy told the court that he was not involved in the incident and claimed that he was innocent.

Healthcare services across West Bengal were impacted for several weeks as protesting doctors at state-run hospitals held a strike against the incident.

The Central Bureau of Investigation is separately looking into alleged tampering of evidence in the case, and alleged corruption at the medical facility. The persons accused in the case include the hospital’s former principal Sandip Ghosh and an ex-officer-in-charge of the Tala police station, among others.