Detectives investigating the murders of two men and youth at a semi-detached house have arrested a fourth suspect.
Hertfordshire Police said a man in his 30s was arrested in London. On Sunday a man in his 20s was detained at Heathrow airport. A spokeswoman said both are being questioned.
Two men arrested in Stevenage, Herts, on Friday have been released on police bail until a later date pending further inquiry, said officers.
Keith Cowell, 52, and his son Matthew, 17, were shot dead at their home in Bishop's Stortford, Herts, six days ago - along with Tony Dulieu, 33, of Billericay, Essex.
A 23-year-old woman and a 54-year-old women were badly hurt in the attack.
Police said a three-year-old girl in the house at the time of the shooting was unharmed.
