NAPERVILLE, Ill. – A mother, her 18-month-old daughter and 4-year-old son died over the weekend in what police called a “suspicious” fire at their suburban Chicago home.

The victims were 32-year-old Nimisha Tiwari, her son, Vakadham Tiwari, and her daughter, Anaya – all of whom were found unconscious in a second-story bedroom Saturday afternoon, authorities said.

“The fire is suspicious in nature,” Naperville Police Sgt. Elizabeth Smith said Sunday. She said the fire was under investigation and declined to discuss what made it appear suspicious.

An autopsy Sunday determined Anaya Tiwari died of thermal burns, and her death was ruled a homicide, according to the Cook County medical examiner’s office. The office would not release the results of the autopsy performed on the child’s mother.

The victims were first transported on Saturday to Edward Hospital in Naperville, where the boy was pronounced dead. His mother and sister were later airlifted to Loyola University Medical Center in Maywood, where they died from their injuries.

Witnesses said the fire did not appear to generate much smoke, and firefighters had it under control within 30 minutes of their arrival.

“It really looked like quite a small fire,” said Avery Spunt, a Naperville resident who was driving nearby at the time. “Shockingly, they started pulling people out, which you don’t expect to see on a Saturday afternoon.”

One neighbor expressed shock at the deaths.

“I wave to every little guy or girl I see, but – God almighty – I never dreamed anything like this would happen,” said Kent Moeller.

The husband and father of the victims was not at home at the time of the fire, according to authorities.

Detectives spoke with the husband and father of the victims and have corroborated information that he was taking business classes Saturday in Chicago at the time of the fire, Naperville police Cmdr. Mike Anders said Sunday.