West Mercia Police have launched an inquiry into how the Shropshire businessman who shot dead his family at Osbaston House, near Oswestry, was allowed to keep an arsenal of weapons after threatening to kill his accountant.
Christopher Foster made a series of death threats against 38-year-old Tim Baker after the collapse of a property deal.
But the failed businessman was able to retain two firearms certificates and in August, 2008, he shot dead his wife, Jillian, 49, and 15-year-old daughter Kirstie at the family's £1.2 million home.
He also shot dead the family's three horses and four dogs before burning the property down with himself inside.
Today the Independent Police Complaints Commission said that a complaint had been received and the matter was 'subject to a local investigation with West Mercia Police.
Police are looking into claims that Foster should have had his firearms licence revoked.
Foster's sister-in-law Anne Giddings, 60, of Wolverhampton, said: "If his guns had been taken away then my sister and my niece would still be alive.
"The fact that he was allowed to keep those guns is absolutely terrible."
Foster had been beset with financial worries in the months before the shootings. An inquest heard that at the time of his death he had assets of £3.1 million but debts of £4.4 million, including three mortgages on Osbaston House.
Coroner John Ellery recorded a verdict of suicide on Foster and of unlawful killing on his wife and daughter.
Leaked police interview transcripts have shown officers were warned he posed a danger as far back as 2006.
His accountant, Mr Baker, told West Mercia Police Foster had made 'several death threats' towards him after the failure of the property deal in Cyprus.
Foster held both a shotgun licence and a firearms licence and kept six shotguns, a single-barrel .22 calibre rifle and a .22 air rifle.
By Sue Austin