Nine families of victims killed at Sandy Hook Elementary and one survivor have brought a wrongful-death lawsuit against the manufacturer of the rifle used in the massacre in Newtown, Conn.

It was two years ago on Sunday when Adam Lanza shot and killed 20 schoolchildren and six staff members using a Bushmaster Model XM15-E2S rifle, which was lawfully purchased by Nancy Lanza, Mr. Lanza’s mother.

The...