Families of children killed at the mass shooting at Sandy Hook Elementary School in Connecticut have filed lawsuits against the estate of the shooter's mother, saying Nancy Lanza was careless and negligent in leaving a Bushmaster AR-15 rifle unsecured in her home.

Adam Lanza, 20, used the assault rifle to kill 20 first-grade students and six adults in Newtown on December 14, 2012.

Adam Lanza was mentally unstable and used his mother's Bushmaster to kill her before he went on the rampage at the school, and then killed himself, said CNN.

Koskoff Koskoff & Bieder, the law firm for the families in the Bushmaster lawsuit, is also representing some families in the cases against Lanza's estate.

CNN said that Nancy Lanza is believed to have had insurance on the home worth more than $1 million.

"Unlike our case against Bushmaster, Camfour (the distributor), and Riverview for the negligent entrustment and marketing of combat AR-15's designed for the military to inflict mass casualties against the enemy to civilians like Nancy Lanza, we expect this claim to be resolved quickly," attorney Josh Koskoff said of the estate lawsuits.

The lawsuits claim that Nancy Lanza kept the rapid-fire Bushmaster in her home "unsecured" and that her "carelessness and negligence" contributed to the pain and suffering of the victims.

The administrator of Nancy Lanza's estate, Samuel Starks, this month asked the courts for an extension to reply and asked that the cases needed to be consolidate, reported CNN.

Read more: http://edition.cnn.com/2015/03/14/us/connecticut-sandy-hook-lawsuits/index.html