THE crucial role CCTV and automatic number plate recognition played in tracking down Sian O’Callaghan’s murderer is  the focus of a new documentary airing in the USA tonight.

See No Evil: Watch Me Disappear, on the Investigation Discovery channel, recounts the 22-year-old’s disappearance and how CCTV and ANPR proved critical in apprehending double-murderer Christopher Halliwell.

Sian disappeared after a night out with friends at the Suju nightclub in Swindon in March 2011.

CCTV captured her leaving the club and walking in the direction of the flat she shared with partner Kevin Reape in Old Town.

Footage also showed a car repeatedly circling the area and pulling up beside Sian shortly after she walked past the Goddard Arms in High Street at 2.57am.

A flash of light then obscured the footage.

When the car pulled away, Sian had vanished.

Although the number plate could not be made out, an specialist said the car was likely to be a dark coloured Toyota Avensis

In what would turn out to be a fundamental break in the case, CCTV showed a police car by chance driving past the car.

The police car had an ANPR camera.

Records showed the registration number along with a minicab advert for Five Star. The green Toyota Avensis was registered to minicab driver Christopher Halliwell.

From then on Halliwell was put under 24-hour surveillance and arrested after being spotted buying an ‘overdose quantity’ of paracetamol at Asda.

He would ultimately lead police to Sian’s body near the Uffington White Horse and to the remains of Becky Godden in Eastleach, Gloucestershire.

Sian’s mother Elaine Pickford, who features in the documentary, said the CCTV and swift identification of Halliwell’s car had proved vital in ensuring the family got justice for Sian,

Elaine, who lives in Old Town, said: “It was a very emotional experience because this documentary solely focuses on Sian and what happened in the days when she was missing

“One of the reasons I wanted to get involved was because of the focus not only on CCTV but ANPR and surveillance.

Without this Sian would have been missing for a lot longer prolonging our agony. The police would also not have known he was a taxi driver or been able to track down Halliwell as quickly.

“I feel I am Sian’s spokesperson.

"On these programmes I want to make sure she is portrayed correctly and I want to raise awareness of how important CCTV is and was in Sian’s case. The police were brilliant.”

See No Evil: Watch Me Disappear will be screened on the Investigation Discovery US channel at 9pm. More information can be found at https://www.investigationdiscovery.com/tv-shows/see-no-evil/