Authorities in the Tihar Central Jail said the five the sixth accused is a juvenile and lodged separately were not even talking to one another or to others in their barracks.
“They just sit and brood,” a jail source said.
The “suicide watch”, officials said, is an alert that the five could possibly harm themselves. A senior officer has been told to monitor their activities.
“We are putting all kinds of check on them to keep them away from (committing) any untoward incident. They are basically on suicide watch,” a jail official told, speaking on condition of anonymity.
Officials said the five, against whom police filed a chargesheet in court Thursday, face no threat from other prisoners.
“They are safe. There is no threat to them inside the jail,” an officer said.
Ram Singh, 33, the main accused and the driver of the bus where the victim was gang-raped and tortured, is in a cell in Jail No. 3.
The sprawling Tihar Central Jail complex has several jails, each identified by a number. While Pawan, 19, and Akshay, 29, are held in Jail No. 4, Vinay, 20, and Mukesh, 26, are in Jail No. 7.
A senior officer has been deputed to oversee the five. Other jail wardens are also on alert.
All the five men and the 17-year-old juvenile, who is in an observation home, are charged with raping the physiotherapy intern in the bus and also thrashing her male friend.
The two victims boarded the bus, which was plying illegally, at Munirka in south Delhi on a cold Dec 16 night.
They were also robbed, stripped and thrown out of the bus some 40 minutes later.
The incident sparked off outrage in India, with protesters demanding the death penalty for the accused and changes in rape laws.
The victim died Dec 29 in Singapore and was cremated in the national capital the next day.
Ram Singh, Pawan, Vinay and Mukesh were arrested within two days of the crime. Akshay and the juvenile were caught Dec 21. Mukesh is Ram Singh’s younger brother.
Authorities said everyone in the Tihar Jail knew about the five.
“They are well known. But they remain quiet when any inmate tries to talk to them,” another officer told.
During a court hearing, Vinay, a gym instructor, told the judge he was not involved in the sexual assault but asked that he should be hanged.