Doris Power was battered to death in her bed

The bodies of Doris Power, 80, her daughter, Amanda, 34, and Amanda's two daughters Katy, 10, and Emily, eight, were all found at their semi-detached home in Clydach, near Swansea, in south-west Wales.

Police believe they may have been murdered with the same weapon.

The BBC's Wyre Davies: "This is one of the most horrific cases South Wales police have ever seen"

A team of 60 detectives is making inquiries into Amanda's background after it was discovered she had both male and female sexual relationships since divorcing her husband, Michael, two years ago.

Detective Superintendent Martin Lloyd-Evans said there was no sign of forced entry at the family's home, which had been set ablaze by the killer.

"There is nothing to suggest that more than one person was involved in this dreadful act," he said.

Inquiries continue into Amanda Power's background

Firefighters discovered the bodies early on Sunday morning when they were called to the house.

Detective Superintendent Lloyd-Evans, heading what is believed to be Wales's biggest-ever murder case, said: "Whoever has done this would be bloodstained and after the crime they have gone home somewhere."

Shouts in the dark

One neighbour said she heard shouts coming from the house shortly before the emergency services arrived at 0430BST.

Gilbert John reports: "More than 60 officers are now involved in hunt for the blood-stained killer

Mr Lloyd-Evans said Amanda and her two children were last seen at about 0030BST on Sunday when they returned to the house after babysitting for a friend.

"Shortly after 4am a neighbour was looking out his window and saw smoke coming from the house.

"He went to the house and tried to gain entry but he could not and called the fire brigade.

"They forced their way into the house and found a fire in the ground floor rooms.

"They then went upstairs with the use of breathing apparatus and a search revealed four bodies.

Motive for killing sought

"There must be a motive for this dreadful incident. It is important that we get to the bottom of this as soon as possible."

Forensic scientists conducted a fingertip search around the family's semi-detached home on Sunday. The front of the house was screened off with plastic sheeting.

Residents in and around the murder scene describe the family as "lovely people". Schoolgirl Lisa Sullivan, 15, babysat for the two girls and said that Amanda Power "lived for them".

On Monday morning, a special assembly was held at the sisters' school, Craig Cefn Parc Primary, as part of a continuing effort to comfort a community in disbelief. Head teacher Jean Healey addressed pupils during a special service.

Speaking before the assembly, she said: "They are going to be coping with their grief because the two little girls were very well known and had lots of friend here. They will be very sorely missed."