Tashfeen Malik: Who was the 'shy housewife' turned San Bernardino killer?

Dr Park Dietz, who has been profiling mass killers for more than 30 years, said he was unaware of any case of a couple erupting in mass violence when the targeted victims were not family members or people perceived to have harmed their child.

“The necessary ingredients for an attack of this kind are to be willing to die that day and an inclination to blame other people, either out of paranoia or a direct grievance,” he said. 

Simon Levshin