Four women suffered burn injuries in a horrific acid attack in the northern Indian state of Uttar Pradesh, police said Wednesday.

The women, all sisters, three of whom were teachers, were walking home from a school in the Shamli district when two men on a motorcycle reportedly made lewd remarks before throwing acid on them. Only the youngest sister, 19, was taken to the hospital with serious burns.

Senior police officer Abdul Hammed told AFP no arrests had been made and the motive behind the attack was unclear.

The incident came as India vowed to protect women following a disturbing trend of sexual assaults. President Pranab Mukherjee signed into law tough new legislation to counter violence against women, including introducing the death penalty for rape.

Women’s rights campaigners say such attacks are often perpetrated by jilted men or relatives using easily available and cheap chemicals, such as “Tezaab” which is designed to clean rusted tools, reports Al Jazeera.

“Acid has become the cheapest and most effective tool for men to attack women in India,” said activist and victim Archana Kumari.

Acid attacks carries a maximum penalty of up to 12 years, depending on the injuries inflicted, but the offence is bailable, according to Al Jazeera.