Grapevine police spokesman Sgt. Robert Eberling said the shooter showed up in the Santa outfit shortly before gunfire erupted and that the family appeared to have been opening Christmas presents. Police responding to a 911 call found four women and three men dead, along with two handguns.
"We think he was just inside there celebrating Christmas with the rest of them and decided for whatever reason that's how he's going to end things," Eberling told The Associated Press.
Investigators worked through Sunday night and into Monday morning, meticulously searching the apartment where the bodies were found, along with vehicles parked outside. Police said they believe the victims were related, though some were visiting and didn't live in the apartment.
Eberling said investigators are piecing together a "family history," but he declined to get into specifics. Autopsies of the shooter and the victims were being done Monday by the Tarrant County medical examiner, but it would probably be Tuesday before their identities were released, he said.
"We're getting a clearer picture, but we're not ready to go on the record with anything until we find out from the medical examiner absolute confirmation of identities and the manner of death," Eberling said.
Roger Metcalf, a spokesman for the medical examiner's office, said the victims have been tentatively identified, but the office couldn't confirm the names because the state driver's license fingerprint database wasn't available on the holiday.
"In addition, we need to locate next of kin before information can be released, and our investigators are working on that as well," Metcalf wrote in an e-mail to the AP.
Late Sunday evening, police intently searched a sport utility vehicle parked outside the apartment. The vehicle is registered to a man who listed his residence as a home two miles away in the neighboring suburb of Colleyville.
Thomas Ehrlich, who lives near the home in Colleyville, told the AP he heard from neighbors that police went to the house Sunday. He said it was his understanding that the man and women who once lived there were estranged.
Records show the couple had financial problems and that their home, most recently valued on the county tax rolls at $336,200, had been sold in 2010 at a foreclosure auction.
Police and firefighters rushed to the Lincoln Vineyards complex about 11:30 a.m. Sunday after receiving a 911 call in which no one was on the other end of the line. Because no one responded on the phone, police went into the apartment, located at the back of the complex.
They found the seven, aged 18 to 60, dead.
Many of the nearby apartments are vacant, and police said no neighbors reported hearing anything on a quiet Christmas morning when many people were not around.