Story highlights Lawsuits against estate of Nancy Lanza say she was careless and negligent

Her son, Adam, used assault rifle to kill 20 first graders and six adults on December 14, 2012

(CNN) 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.

The younger 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.

At least eight lawsuits have been filed in Connecticut courts since January on behalf of relatives of 16 people killed in the massacre -- four adults and 12 children. Court documents also list two people who suffered serious injuries.

In December, the parents of nine children killed at the mass shooting, along with one teacher who survived, filed a separate lawsuit against the businesses behind the Bushmaster rifle.

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