The negligence and wrongful death lawsuit says the gunmaker should not have made the military-style rifle available to the public.
The families of nine of the 26 people killed by a gunman at the Sandy Hook Elementary School in Connecticut have sued the manufacturer of the rifle used in the attack.
The negligence and wrongful death lawsuit also names the distributor and seller of the Bushmaster AR-15 rifle.
The families claim the rifle should not have been made available to the public because it is a military weapon unsuited for civilian use.
Fake image of Adam Lanza
“The AR-15 was specifically engineered for the United States Military to meet the needs of changing warfare,” attorney Josh Koskoff said in a release.
“In fact, one of the Army’s specifications for the AR-15 was that it has the capability to penetrate a steel helmet.”
Adam Lanza shot his way into the elementary school in Newtown on 14 December 2012 before killing 20 children and six educators with the rifle. He then fatally shot himself with a handgun as officers arrived.
Images released by investigators showed spent AR-15 magazine clips and empty shell casings that followed the gunman’s path of destruction.
In addition to Bushmaster, the families have named Camfour, a firearm distributor, and Riverview Gun Sales, the store where the rifle was purchased in 2010.
None of the companies named in the lawsuit have commented.
Bill Sherlach, whose wife, Mary, was killed in the attack, said although he believes in the Second Amendment, which protects Americans’ right to keep and bear arms, the gun industry needs to be held to “standard business practices”.
“These companies assume no responsibility for marketing and selling a product to the general population who are not trained to use it nor even understand the power of it,” he said.
The massacre in small Connecticut town shocked the country and rekindled a debate on gun control.
A news conference is to be held later on Monday, with several US lawmakers scheduled to attend.