Dallas (CNN) -- Autopsies were being conducted Monday on seven family members who were fatally shot by a man dressed as Santa Claus 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 said 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."
The shooter is believed to have been related to the family by marriage, he said. Officers were investigating whether divorce was an aspect of the shooting, he said.
The identities of the victims were expected to be released Tuesday, along with autopsy results, Eberling said.
Police said the group apparently had just finished opening their gifts when the shootings occurred in the living room of the apartment in Grapevine, about 25 miles northwest of Dallas.
Authorities responded to the address Sunday after receiving a 911 call from the apartment, but hearing only an open line, police said in a statement.
The seven bodies -- four female adults and three adult males -- were found when officers entered the apartment. "All victims appear to have gunshot wounds," said the statement.
The victims range in age from 15 to 58 or 59 years old, Eberling said Monday.
"The biggest thing we want to stress is we don't believe this person had barged his way in," Eberling said. "It appears they had all gathered in the living room for your garden-variety Christmas celebration."
CNN's Carma Hassan, Chuck Johnston, and Nick Valencia contributed to this report.