Authorities are investigating whether a mobile-home fire that killed two adult sisters and two school-age girls Tuesday was arson or an accident.

A K-9 dog detected a type of hydrocarbon liquid, a flammable liquid, in the wreckage of the home, said Deputy Fire Chief Ron Simmons of Snohomish County Fire District 4.

"The investigation is going to identify whether it was truly arson or whether it was an accidental fire," Simmons said.

Firefighters found the bodies after extinguishing the flames at a mobile home park in this community about 25 miles north of Seattle.

The family had just moved in over the weekend, said Sherree Simpson, who manages the 44-unit Snohomish Mobile Home and RV Park.

"Everybody's upset," said Simpson, who didn't know the family well. "It's devastating."

Names of the victims were not immediately available.

The adult sisters, 28 and 25, 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.

The two girls, ages 7 and 4, were the children of one of the sisters, Simmons said.

Snohomish police are investigating with help from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and the Washington State Patrol, Simmons said.

When firefighters arrived about three minutes after the blaze was reported at 6:57 a.m. Tuesday, the home was engulfed in flames. The smoke and flames could be seen from the fire station about a block away, Snohomish police Sgt. Jeff Shelton said.

The mobile home had an electric wall heater and a wood-burning stove, and investigators will try to determine whether either were on at the time, he said.

The family had recently picked up a school registration packet, said J.Marie Riche, a spokeswoman with the Snohomish School District.