The families of two victims of the Washington snipers are suing the gun-maker and gun shop where the weapon was bought.
The suit alleges their negligence led to innocent people losing their lives.
The legal action accuses the owners of the Shooter Supply shop of gross negligence in allowing the sniper suspects, John Alan Mohammed and John Lee Malvo to acquire a gun, even though they were legally prohibited from doing so.
It accuses the gun-maker, Bushmaster, of being equally remiss in continuing to supply the weapon to the shop, even though government audits of the store had revealed hundreds of missing guns.
'Misconduct'
The action is supported by the Brady Center, a group that campaigns to prevent gun violence.
"No lawsuit we have ever filed more dramatically illustrates how the misconduct of members of this industry can cost innocent lives," says the Brady Center's director Denis Henigan.
A spokesman for Bushmasters says it did nothing wrong and sold the rifle legally to a firearms dealer.
The lawsuit comes just a day after a court ruled that 17-year-old John Lee Malvo can be tried as an adult for his alleged role in the sniper killings.
The ruling means that if convicted, he can be sentenced to death.