Observing their demeanour, police said they would verify whether the men have criminal records.

"Their involvement in cases not yet reported might surface," another investigator said.

All the four men have been held in connection with the brutal rape and torture of the woman Sunday night in a moving private bus.

The girl and her male friend boarded the bus, which had tinted glasses and windows.

The bus was driven through the bustling south Delhi areas of Munirka, Vasant Vihar and Mahipalpur as the six men raped and tortured the girl and beat her friend, using iron rods and other weapons.

The couple was stripped, robbed and thrown off the bus near Mahipalpur.

The brutality of the gang-rape shook the nation and people came out on the streets to demand action against the accused.

The beastly manner of the crime led to a furore in parliament, with member after member demanding justice for the girl and change in laws to prevent such crime.

The victim is undergoing treatment at Safdarjung Hospital. Her condition is still critical.

Investigators said the accused were insolent, behaving as if the crime has not affected them. Ram Singh, the main accused who was arrested Monday, goes about his routine unmoved by the plight of the woman he savaged, policemen said.

Police have yet to come across a case lodged against Ram Singh, but investigators say they will not be surprised if it turns out that the man has a criminal history.

Ram Singh's elder brother Rajesh was convicted in a robbery case and is currently lodged in Tihar jail; his younger brother, Mukesh, is a co-accused in the gang-rape case.

It was on Ram Singh's instance that the police were able to nab three others involved in the case, including his brother. Mukesh, however, has claimed that he had thrashed the 28-year-old male friend who accompanied the woman, but had not raped the woman.

Mukesh had agreed to take part in the identification parade Thursday at Tihar jail.

The main witness, the woman's male friend, identified him as one of the culprits.

The woman's friend, a software engineer, had attempted to intervene and save her, but was beaten up by the six men.

Police are still on the lookout for two other accused.

"Our various teams are working to nab the other two accused who are absconding. The teams are camping in Bihar, Rajasthan, Haryana and Uttar Pradesh," said a police official.

Police has sought close-circuit TV footage from the Select City Walk Mall, Saket. The woman and her friend had boarded a bus after watching a movie at the mall Sunday night.

The accused had burnt the clothes of the victims, and washed the bus in a bid to destroy evidence.