A missing hubcap on a wheel and a white bus - these were the clues that led the police to zero in on the vehicle in which a 23-year-old physiotherapy intern was gang-raped in Delhi on December 16, 2012.
The woman, who succumbed to injuries as a result of the assault, was waiting with her male friend at the Munirka bus stand. A white bus with yellow and green stripes came near them with a young boy was shouting out for passengers going towards Dwarka and Palam Mod.
They boarded the bus.
Inside the bus, her male friend was attacked while the woman, who came to be known as 'Nirbhaya', was gang-raped by six men who later threw them off the bus - an incident that shook the conscience of the country.
Tracing the bus was the first step in the investigation. One of the investigating teams woke up Delhi Transport Department officials in the early hours to get details of all white buses registered with them, former Delhi Police commissioner Neeraj Kumar has written in his book.
Soon, the police had the details of 320 white buses. Their search was further streamlined after some other teams scanned the CCTV footage from all hotels and guest houses on the national highway.
"The police teams noticed a white bus with 'Yadav' painted on its side in the CCTV footage of Hotel Delhi Airport. The bus was seen coming from Delhi, seemingly heading for Gurugram, but within nineteen minutes, at 9.53 pm, it reappeared on the screen, heading again in the same direction," Mr Kumar has written in his book.
The footage was shown to Nirbhaya's friend who identified the bus since its front left hubcap was missing.
Subsequently, police found out that the bus was owned by Dinesh Yadav of Uttar Pradesh's Noida. He confirmed that his bus had red seat covers, yellow curtains and Yadav written on its side, according to the book.
Dinesh Yadav told the investigators that the bus driver's name was Ram Singh, who lived in Guru Ravi Das Camp in RK Puram. From the camp, the bus was recovered and Ram Singh was also arrested.
After the recovery of the bus, another challenge confronted the police - that of keeping the vehicle safe as evidence.
According to investigators, parking the bus inside the Vasant Vihar police station was not a safe option since the station was witnessing protests against the rape and there was a danger that the bus could be vandalised.
The investigators got the bus parked at the Thyagraj Stadium parking lot and forensic teams were called there for examination.
This led to the rest of the investigation and arrest of all the accused.