The suit named gunmaker Bushmaster, a distributor and the local retailer that sold the weapon used by 20-year-old Adam Lanza to kill 20 first-graders and six educators in a Dec. 14, 2012, attack at Sandy Hook Elementary School.
The lawsuit contends that the AR-15 rifle Lanza used in his attack was designed to be a military weapon, had "little utility" for civil purposes and thus should not have been in Lanza's hands.
"There is one tragically predictable civilian activity in which the AR-15 reigns supreme: Mass shootings," the lawsuit said.
Families of nine victims and a 10th person who was wounded filed the suit in Connecticut Superior Court in Bridgeport. The suit seeks unspecified monetary damages.
Bushmaster did not immediately respond to a request for comment.
Lanza, who began the shooting spree by killing his mother at their home, ended his rampage by turning his gun on himself as he heard police approaching.