There is a loneliness epidemic in the United States -- and experts told ABC News it may be triggering violence.

In California, there have been three shootings in as many days, tied to a perpetrator who may have exhibited signs of social isolation and/or violent behavior, according to authorities.

In Monterey Park, police documents revealed the 72-year-old suspect had been divorced from his wife since 2006, lived alone in Hemet -- about 30 miles Southeast of Riverside -- and was angry and resentful.