They sued the estate.
They may have sued the estate, also. But, you can't find someone responsible of negligence without naming them in the suit as well. I haven't seen a copy of the suit, but it's highly unusual that a perpetrator's family member (in this case spouse) be sued, especially when they aren't named by LE as an accomplice to the crime.
Like I said, the perpetrator's estate is going to bear no fruit. The only chance the victim's attorney's have at a jury award is to sue the family. It's despicable, and it's not reflective of a well-functioning tort system.