All four people found dead at a property near Norwich had injuries, police have said.
Two young girls, a 45-year-old man and a 36-year-old woman were found dead at a home in Allan Bedford Crescent, Costessey, Norfolk, shortly before 7am on Friday.
All four were part of the same family, according to Detective Chief Inspector Chris Burgess of Norfolk Police.
"Three of the people are believed to reside at the address and another one not permanently," he said.
"The man and two children lived at the address, but the woman involved was visiting and did not live at the property."
He said post-mortem examinations would be held to establish the cause of their deaths.
Police forced their way into the property following a call from a member of the public. They also confirmed officers attended the property on 14 December last year in relation to a missing person inquiry.
"We are still working to understand the full circumstances which led to the deaths of these people, however at this time we believe this to be an isolated incident," Mr Burgess added.
"I recognise the shock and upset this incident will cause in the local community and we will have increased patrols in the area across the weekend."
In an earlier statement, he described the incident as "very distressing and tragic".
"I would stress to the community, let's be very aware of what we share on social media without confirmed information," he said.
"We don't want to cause any undue distress or give false information that might actually hamper the investigation rather than support it."
A cordon is in place and detectives from Norfolk and Suffolk police forces are leading the investigation.
Officers are expected to remain in the area for some time.