SNOHOMISH, Wash. - Investigators say a mobile home fire that killed two girls, their mother and their aunt this morning may have been arson.

Officials say an arson dog got a hit on an accelerant at the site, which suggests the possibility of a crime scene. The Bureau of Alcohol, Tobacco and Firearms and the Washington State Patrol were called in with special equipment and training.

"Right now, we don't have the facts to call it an arson case. We have the facts that we think are necessary to secure a search warrant," said Snohomish Police Chief John Turner.

The home was fully involved in flames when firefighters arrived about 7 a.m. at the Snohomish Mobile Home and RV Park at 1330 Ave. D.

Firefighters arrived just three minutes after the fire was reported.

The fire department is only a block away, but like most mobile home fires, Simmons said, it's usually over quickly. It was not clear at first that anyone was inside.

"Once the main fire was knocked down and the crews were going in to do the mop up, that's when we discovered the bodies," said Simmons.

The children are ages 7 and 4. Their mother was 28 and her sister was 25.

"They wanted to advance themselves," said Abel Montano, the girls' uncle, speaking through a translator. "They were paying a lot of rent ... saving money to buy a trailer."

The sisters arrived in the U.S. from Guadalajara, Mexico, four years ago, and lived in an apartment in Snohomish before moving to the mobile-home park, Michaela Villa SeInor, the women's cousin, told The Seattle Times.

Park manager Sherree Simpson said sisters had just bought the mobile home and moved in on Friday. One of them had two children.

"They were just still kind of in the process of moving in. They were moving stuff in trucks over the weekend," she said.

Simpson said the park has 44 units.

"We've never had anything like this happened and it's a pretty close-knit community so everybody really cares about people, so pretty upset," said Simpson.