Two girls found hanging earlier this year in northern India had committed suicide and were not gang-raped and murdered as previously thought, federal investigators said.

After a months-long investigation, India’s Central Bureau of Investigation (CBI) said on Thursday that it "has concluded that it is a case of suicide".

"Based on 40 scientific reports, CBI has concluded that the two minor girls had not been raped and murdered as had been alleged in the (initial police report)," the agency said in a statement.

The cousins, aged 12 and 14, were found hanging from a mango tree in an impoverished village in Uttar Pradesh state in May, reigniting international outrage over crimes against women in India.

The girls' families said they were raped and lynched by men from a higher caste after going into the fields at night to relieve themselves because their ramshackle homes did not have toilets.

Police arrested five men over the case after the families accused authorities of failing to act because they came from a lower caste. All five were later released without charge.

The case had sparked fresh debate about India's treatment of women, less than two years after public anger erupted over the fatal gang-rape of a student on a moving bus in Delhi.

That attack led to tougher laws for rapists and other sex offenders and new policies for police in dealing with the large numbers of assaults against women.

Women’s rights activist were sceptical about the latest decision in the Uttar Pradesh case and have urged officials to reinvestigate.

"It must be taken seriously, decisions should not be taken in haste. The CBI should revisit its report and culprits must be punished," Delhi Commission for Women chairwoman Barkha Shukla told reporters.