The Stirlands had refused police protection
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Three men are due to be sentenced for plotting the murder a couple on the Lincolnshire coast.
John and Joan Stirland were shot in August 2004 in revenge for a murder committed by her son.
Michael McNee, 21, of no fixed address, John Russell, 29, of Northcote Way, Nottingham and a 39-year-old defendant, who cannot be named, were found guilty.
Five other men were cleared on the same charge after a trial at Birmingham Crown Court.
Police said they believe there were more people to be brought to justice.
Safety 'compromised'
Lincolnshire's Assistant Chief Constable, Peter Davies, said: "We are going to review the evidence, we are going to review the investigation and see if there are other lines of enquiry we can follow.
"There are people still at large who were involved to a greater or lesser extent in the murders of these two people and we need to bring them to justice."
The Stirlands were killed in revenge for a murder carried out by Mrs Stirland's son, Michael O'Brien, in Nottingham in 2003.
Shane Bird, 39, of Carlton Hill, Nottingham; Kevin Holm, 38, of Cliff Road, Carlton; Lanelle Douglas, 20 and Andrew McKinnon, 22, both of no fixed address and a 40-year-old defendant, who cannot be named for legal reasons, were cleared of the charges.
The Independent Police Complaints Commission is now investigating the communication between Nottinghamshire and Lincolnshire police on the day of the killings and whether that compromised the Stirland's safety.