Seven people believed to be related were gunned down as they cleared up after opening their Christmas gifts at a Texas apartment.

Authorities in Grapevine said they believed the gunman was among the dead, but got a warrant before conducting a full search on the small chance that it was otherwise.

Four women and three men, aged 18 to 60, were found in an adjoining kitchen and living room area when police entered the apartment at around midday, said Sgt Robert Eberling.

Two handguns were found near the bodies in the apartment that was decorated for Christmas with a tree, he added.

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

The victims have not yet been identified, but Sgt Eberling said it appeared they all died of gunshot wounds. He said authorities did not know what sparked the incident.

Lt Todd Dearing of Grapevine police said investigators believed that all the victims were related, but that some were only visiting. He said police were looking for other relatives to inform.

"Seven people in one setting in Grapevine, that's never happened before. Ever," Lt Dearing said.

He said police were performing a "meticulous" search of the apartment and he expected them to be on the scene for many hours.

Police and firefighters first rushed to the Lincoln Vineyards complex after receiving an open-ended emergency services call at about 11.30am.

"There was an open line. No-one was saying anything," Sgt Eberling said.

Police went into the apartment, in a middle-class, suburban neighbourhood of Grapevine, not far from the Fort Worth neighbourhood of Colleyville. The apartment is at the back of the complex, overlooking the athletics fields of Colleyville Heritage High School.

But many of the nearby apartments are vacant and police said no neighbours reported hearing anything on a quiet Christmas morning when many people were not around.

Jose Fernandez, a 35-year-old heavy equipment mechanic who moved to the complex with his family about six months ago, said he always felt safe in the area, but was now afraid to let his 10-year-old son play outside.

"This is really outrageous especially on Christmas," said Mr Fernandez, who was visiting his family for and returned to find several police cars parked outside his home.

"This has shocked everybody. It has scared everybody. I guess something like this can happen anywhere, but seven people dead. It's just very scary," he added.

Sgt Eberling said the area was fairly quiet, and this would be the first murder in Grapevine since 2010.

Christy Posch, a flight attendant who moved to the complex about six months ago, said: "It's all families. That's why I moved here. No burglaries, no nothing."