In this Oct. 28, 2018, file photo a Pittsburgh police ...

Photo by Associated Press | published: October 31, 2018

In this Oct. 28, 2018, file photo a Pittsburgh police officer walks past the Tree of Life Synagogue and a memorial of flowers and stars in Pittsburgh in remembrance of those killed and injured when a shooter opened fire during services Saturday at the synagogue. The perpetrators of mass shootings often provide a treasure trove of insight into their violent tendencies on social media. But the information is not always seen by law enforcement until after the violence is carried out.