A 15-year-old boy returned home from walking his dog to find his mother had been beaten to death.
The body of Eve Howells, 48, a history teacher, was found lying on a settee at the family's bungalow in Huddersfield, West Yorkshire, on Thursday night.
Three men and a woman were arrested yesterday and are helping police with their inquiries.
A post-mortem revealed Mrs Howells had died from severe head injuries caused by blows from a hammer or similar instrument, although no weapon has been found.
Detective Superintendent Gary Haigh, leading the murder inquiry, said he suspected Mrs Howells died at the hands of burglars. Cash worth pounds 100 was missing from the bungalow and rooms had been searched.
"It was a vicious attack after which a young boy found his mother brutally murdered," he said. "It left a family completely devastated."
Mrs Howells was alone in the bungalow when the attack happened. Her husband, David, was at a local pub and the 15-year-old son and his 14-year-old brother had taken their dog for a walk.
When the boy found his mother he rushed next door to neighbours to raise the alarm.
A teacher at Newsome High School, Huddersfield, Mrs Howells was due to return to school yesterday to prepare for the new term starting on Monday.Reuse content