They believe the shooting spree by 26-year-old Devin Patrick Kelley in Sutherland Springs was driven by a rift in family relations, adding that the mother of Mr Kelley's wife attended the First Baptist Church.
Texas Department of Public Safety Regional Director, Freeman Martin also said he didn't believe the incident stemmed from a religious or racial motive.
Meanwhile, United State Vice President Mike Pence announced that he and his wife, Karen, would travel to Texas on Wednesday and meet bereaved relatives, the injured and emergency service personnel caught up in the shooting.
The attack, which left 20 other people wounded - including ten who remain in a critical condition in hospital -, is the worst mass shooting Texas has ever experienced.
Victims are now known to have included people who ranged in age from 18 months to 77 years old, and multiple members of some families.
Authorities say Mr Kelley had been confronted on at least two occasions in the last five years about domestic violence, including an assault on his ex-wife and a child that ended his Air Force career.
The US Air Force has started a review after failing to provide information to authorities that would have stopped him from legally buying firearms.
Evidence suggests Mr Kelley died of a self-inflicted gunshot wound after he was chased from the church by armed bystanders and later crashed her car.