Police said a man in his 40s was detained in London on suspicion of assisting an offender. A man in his 30s was arrested in London on Monday and on Sunday a man in his 20s was detained at Heathrow Airport.

A spokeswoman said all three were being questioned by police.

Two men arrested in Stevenage, Herts, on Friday have been released on police bail pending further inquiries.

Keith Cowell, 52, and his son Matthew, 17, were shot dead at their home in Bishop's Stortford, Herts, on August 28 - 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.