As ITV's new drama A Confession hits our screens - here's how the real-life events unfolded.
1 JANUARY 2003
Becky Godden, 20, is last seen alive getting into a taxi outside a club in Swindon after a night out with friends.
19 MARCH 2011
Sian O’Callaghan, a 22-year-old office worker, vanished on a night out with friends in Swindon's Suju nightclub.
Her boyfriend Kevin Reape raised the alarm - and soon Detective Superintendent Steve Fulcher was leading the inquiry into her disappearance.
21 MARCH 2011
Kevin, and the rest of Sian's family, make an emotional appeal for her return.
A volunteer army of hundreds turn out to help with searches of the nearby Savernake Forest, where Sian's phone had connected with a nearby mast.
23 MARCH 2011
CCTV of Sian’s last-known moments led Det Supt Fulcher to taxi driver Chris Halliwell.
Halliwell was put under 24-hour surveillance. Surveillance teams watched Halliwell cleaning the rear passenger seat of his car with chemicals, they saw him throw away car seat covers in a wheelie bin across town and they saw him burning material in the countryside in the middle of the night.
They then lost him.
24 MARCH 2011
Officers arrest Halliwell, after he buys a quantity of drugs from a supermarket - Det Supt Fulcher becomes worried Halliwell will take his own life and the route to Sian would be lost.
Sitting in headquarters, Fulcher ordered the arrest. Officers stopped Halliwell as he got into his car at a Swindon supermarket. Halliwell was cautioned under the Police and Criminal Evidence Act (PACE.) He had a right to remain silent.
And he refused to cooperate. Halliwell told the arresting officers he wanted to go to a police station and get a solicitor.
It was at this point that Fulcher made the decisive gamble.
Rejecting advice from both junior and senior officers, Fulcher ordered that Halliwell be taken to Barbury Castle, an Iron Age hillfort South of Swindon.There the two men met.
Fulcher said he was using so-called ‘Emergency Powers’ when there is a threat to life - usually reserved for terrorist cases.
He should have cautioned Halliwell again, but refused to.
Fulcher pleaded with Halliwell. ‘I know you did it’ the detective told Halliwell. He also said the taxi driver would be vilified in the press within the hour and he ‘should do the right thing.’
Halliwell turned to Fulcher and said: ‘Do you have a car?’
Police teams were directed by Halliwell to a roadside verge, near Uffington in Oxfordshire, where he said he had left Sian’s body, after killing her.
There was no more threat to life. Sian was dead.
But what followed was another astonishing admission by the taxi driver as he admitted killing years before. He even showed detectives the field in Gloucestershire where he had dumped the body of a young Swindon woman years before.
Again, there was no threat to life. The first victim had been dead for eight years.
According to PACE rules Fulcher should have read Halliwell his rights this three times on that afternoon. He failed to, knowingly.
25 MARCH 2011
Remains of a second woman were found at the site pointed out by Halliwell.
28 MARCH 2011
Halliwell appears in court in Swindon Magistrates charged with the murder of Sian O'Callaghan.
5 APRIL 2011
The remains of the second woman are identified as Becky Godden-Edwards.
11 APRIL 2011
Becky's tearful mother Karen gives a press conference confirming the identification
19 OCTOBER 2011
However, as Det Supt Fulcher had not cautioned Halliwell as he should have under the Police and Criminal Evidence Act (PACE), a charge for Becky's murder was effectively dropped.
23 JANUARY 2014
Det Supt Fulcher is found guilty of gross misconduct, but is handed a written warning, allowing him to keep his job.
16 MAY 2014
New lead officer Detective Superintendent Sean Memory takes over the case and gets result immediately - searching a lake to find possessions belonging to Sian.
10 NOVEMBER 2014
A search is conducted of Halliwell's former home in Swindon. Teams find nothing.
18 FEBRUARY 2015
Det Supt Memory visits Halliwell in prison - but declines the deal Halliwell offers.
15 JUNE 2015
A spade found in Halliwell's shed in 2011 was retested for soil samples, matching the field in which Becky's remains were found.
5 SEPTEMBER 2016
The trial begins into Becky's murder begins.
19 SEPTEMBER 2016
In his sentencing remarks Mr Justice John Griffith Williams tells Halliwell he believes he has "no doubt that you are a self - centred and domineering individual whow ants his own way. You are both calculating and devious."