Shehla Masood, who was campaigning against plans to open a mine when she was killed.

For many of her 38 years, Shehla Masood had campaigned tirelessly against corruption. Glamorous and combative, she had embraced India's Right to Information Act with gusto, rattling out applications in all directions, exposing wrongdoing at the highest levels of Madhya Pradesh state where she lived, upsetting many powerful people with a great deal to lose. Judges, police and politicians from the local ruling BJP party had all come into her sights and been exposed for misusing public cash.

In recent months, she had turned her attention to mining conglomerateRio Tinto's plans to extract 37 million tonnes of diamond-bearing ore from land in one of the finest strands of teak forest in the country.

Then, on 16 August, Shehla was found dead in her car outside her home in a prosperous area of Bhopal, with a single gunshot to her neck. More than a month later, the investigation has hit a brick wall. Even the offer of a £7,500 reward – an enormous sum for India – has failed to elicit a single witness to a killing that took place in broad daylight in a busy street.

It is Tuesday morning in that same respectable street in Bhopal. A large khaki tent is pitched opposite Shehla's house. Four police officers, posted to guard her family, sprawl inside on charpoys, fast asleep. The road leads to a large slum, whose residents pass regularly in front of the house, much as they must have done on the morning she died.

It was Shehla's father, Sultan Masood, who found her lying with her head back in the front seat of her little silver Hyundai Santro car. "I called: 'Shehla, Shehla', but she didn't speak. I took some water and splashed it on her face and then her dupatta [scarf] slipped down and I noticed the black hole in her neck. I started screaming: 'Somebody has killed my daughter, someone has shot my daughter.'"

It is almost inconceivable that no one saw the killer or heard the shot, but Shehla's fate appears to have been a warning to others to keep silent.