- Video report by ITV News West of England Correspondent Rupert Evelyn
Police are carrying out excavation work at the former Swindon home of double murderer Christopher Halliwell.
Halliwell, 53, murdered 20-year-old Becky Godden in January 2003 and 22-year-old Sian O'Callaghan, in March 2011.
He was given a whole-life order, meaning he will never be released, in 2016 after being convicted of murdering Miss Godden.
After his sentence, police said there was a "distinct possibility" that he is a serial killer, particularly as there was an eight-year gap in the murders.
Police began searching the gardens and garages of two properties on the street where Halliwell lived, on Monday, and the excavation work is expected to last five days.
Having seen Halliwell in court when he was on trial for murdering her daughter, Elaine Pickford, the mother of Sian O'Callaghan said she believes he is a "psychopath".
"On the surface I'm sure he had no problem fitting into everyday life, because that's how psychopaths operate, they are not people that make themselves stand out," Ms Pickford said.
"They are people who are very good at blending in."
A spokeswoman for Wiltshire Police said: "Work at the addresses in Broad Street is expected to last five days. This intelligence-led activity is part of an ongoing investigation."
Detective Chief Inspector Jeremy Carter, leading the investigation, said: "Whilst we do not have plans, at this stage, to search inside the properties there will be some disruption to the occupants.
"I would like to make it clear that those living at the addresses have no involvement in the investigation and we thank them for their understanding and patience."