Crime scene photographer is silhouetted against blood splattered window blinds in an apartment in Grapevine, Texas, December 25, 2011.[Photo/Agencies]

GRAPEVINE, Texas - A gunman who killed six people and himself at a family Christmas celebration outside Dallas was dressed in a Santa Claus suit when he opened fire, police said on Monday.

Authorities continued their search for clues in Grapevine, a Dallas suburb dubbed the "Christmas Capital of Texas," to explain the Sunday murder-suicide rampage that left the seven shot dead among unwrapped holiday presents.

The dead -- four women and three men ages 15 to 59 - were found on Sunday morning in an apartment living room by police answering a voiceless 911 emergency call, authorities said.

"By all appearances, they're all part of the same family," said Sergeant Robert Eberling of the Grapevine police department, adding that some were related through marriage.

Two pistols were recovered from the home, said Sergeant Robert Eberling of the Grapevine police department, who called it a "gruesome crime scene" and the worst outburst of gun violence in the town's history.

The last homicide in the town was in June 2010.

Authorities were waiting on autopsy reports before releasing identities, a potential motive and details on what exactly happened inside the home, Eberling said.

"We have a petty good idea who these folks were, and we're trying to work through contacting other family members so we can better piece together what took place and why it took place," he said.

Eberling said the shooter was dressed in a Santa Claus suit but gave no details.

Grapevine police Lieutenant Todd Dearing said the victims' ages were 15, 19, 22, 55, 56, 58 and 59. The victims in their 50s were two couples, Dearing said.

The shooter was one of the older men, he said.

Circumstances of the shooting remained sketchy, but Eberling said it appeared as though the bloodbath unfolded during a family holiday celebration.

No one was found alive by police arriving at the home, he said.

A community of about 46,000 people some 20 miles (32 km)northwest of downtown Dallas, Grapevine is known for its wine-tasting salons. It was proclaimed by the state Senate as the "Christmas Capital of Texas" for its abundance of annual holiday-season events.

"This is obviously a terrible tragedy," Mayor William Tate said Sunday night in a statement.

"The fact that it happened on Christmas makes it even more tragic."