Autopsies will be conducted Monday on seven family members who were shot - apparently by one of their own - at a Christmas gathering, police said.

"The information we have is that this is a family, some of them related by marriage but mostly by blood," Grapevine, Texas, police Sgt. Robert Eberling said Monday. "They have lived in other areas in the (Dallas) metroplex and they came yesterday for a Christmas gathering."

Authorities believe the shooter "was one of the individuals inside the apartment that was also deceased," Eberling said. "We know he is a family member ... we are working to find out why exactly this took place. We contacted other family members to piece this together."

Police said Sunday the group was either in the process of opening Christmas gifts or had just finished doing so when the shootings occurred in the living room of the apartment in Grapevine, about 25 miles northwest of Dallas.