Seven people were found shot to death Christmas Day in an apparent murder-suicide, Grapevine police said.

Officers had rushed to the Lincoln-Vineyards Apartments in the the 2500 block of Hall Johnson Road after no one responded to an operator when someone called 911 from inside the apartment at about 11:30 a.m.

Police found the bodies of four women and three men ranging in age from 15 to 58 years old when they kicked in the door.

Sgt. Robert Eberling confirmed Monday that the suspected shooter, who was among the dead inside the apartment, was wearing a Santa Claus costume. Police said the middle-aged man did not live at the apartment but was visiting.

"It appears they had just celebrated Christmas. They had opened their gifts," Eberling said.

Police said the victims and two handguns were found in the living room and kitchen. Police said unwrapped presents and wrapping paper were scattered throughout the apartment's first floor.

Grapevine police told NBC 5 they believe they know the motive behind the Christmas morning massacre.

"We do have a good idea of what took place and why," Eberling said.

He declined to reveal the suspected motive until autopsies of the victims are complete.

Investigators said they had identified the victims but were waiting for medical examiners to confirm the identities and notify family members. Police said all of the victims were related by blood or marriage but did not all live at the apartment.

Neighbors said the victims were the Yazdanpanah family.

Allison Baum, a friend of of Nargis Yazdanpanah, said her friend's parents had separated earlier this year. She said 19-year-old Nargis Yazdanpanah lived with her mother and her 15-year-old brother, Ali.

"I would pick her up down the street, and when they moved at the apartment, we'd go to school together, had classes," she said. "She was kind to everyone."

Baum and her mother, Terri, said the family was still close.

"It seems like their whole existence was about family, so it's utterly shocking to me," Terri Baum said.

Neighbors said they were shocked by the shootings.

"It doesn't matter what day of the year this is, never should this sort of thing happen," Pam Lyon said.

"We've been living here for the last four years, and we're never seen this before," Abdul Rahmanshibji said. "This neighborhood is very good, very quiet; never had a problem."

The apartment where the incident occurred is at the back of the complex, overlooking the athletic fields of Colleyville Heritage High School.

"I have been with this police department for 12 years, and this is probably one of the most tragic things that I've certainly been involved with," Eberling said.

Grapevine police said the shootings are the city's first homicide of the year.

NBC 5's Ben Russell and Ray Villeda contributed to this report.