West Suburban Bureau

CBS 2's Mai Martinez, Dana Kozlov, and the Naperville Sun contributed to this report.
NAPERVILLE (CBS) ― There are several unanswered questions Sunday evening about a fire that killed a west suburban mother and her two children. CBS 2's Dana Kozlov reports, Naperville police have a long way to go in solving the case.

And that's why Naperville investigators were back at the scene of the Sunday, spending hours at a charred Naperville house at 1960 Nutmeg Lane, collecting evidence. The fire started Saturday afternoon, ultimately taking the lives of 32-year-old Nimisha Tiwari, her four-year-old son Vakadham, and 18-month-old daughter Anaya.

Firefighters found all three victims unconscious in a second-story bedroom in Naperville's Cinnamon Springs subdivision after someone driving by the home saw smoke and called 911 Saturday afternoon.

Officials said the three victims were initially taken to Edward Hospital, where the 4-year-old boy was pronounced dead. His mother and sister were later transported to the burn unit at Loyola University Medical Center.

All three victims suffered severe burns and smoke inhalation.

The mother died later Saturday night and her daughter died early Sunday morning. An autopsy ruled the death of the 18-month-old, a homicide. She died of thermal burns, according to the Cook County Medical Examiner's office. Autopsy results for the mother are pending.

Police said Nimisha's husband was not home at the time of the blaze and has been very helpful in their investigation.

"The father, Mr. Tiwari has been ruled out as a suspect in this incident," said Naperville police commander Mike Anders.

Family members were too upset to talk about the deaths of their loved ones Sunday morning. Neighbors say it's heartbreaking to hear all three died.

"They were a happy family. That was my impression. We were really shocked when we heard about it," said next door neighbor Steven Naday.

Neighbor Ooni Nair said the children were "just playful kids. We had seen them a couple of times. They were taking the kids on the stroller."

Naperville police and fire investigators were on the scene Sunday morning gathering evidence and looking into the cause of the blaze.

"We've ruled out that there was an accidental fire," said Anders.

But whether police are just focusing on the mother or whether they're searching for other suspects remains unclear. Detectives did canvass the neighborhood, talking to neighbors and they did ask question at a nearby gas station.

An employee of another gas station said police also came there, showed two photos and asked if wither person had purchased gasoline. Police won't say if there were any family problems, but neighbors say the Tiwaris seemed happy.

"Good neighbors, definitely," said Chandrakanth Shet, a friend of the Tiwaris.

"Certainly a great deal of evidence has been removed, and the home is still secured," Naperville Police Sgt. Betsy Smith said. "There is a very active investigation going on. The fire in itself appears suspicious in nature. Like with all fire investigations we are working with the arson task force to really pinpoint not only the cause of fire, but the death of three victims."

Neighbors agreed with the assessment that the fire was suspicious. They say most of the houses in the subdivision have similar layouts and they're all equipped with smoke detectors.

"It is really very strange because it happened during the day. It's very hard to imagine that they didn't hear the fire alarm," Naday said.

"There were smoke detectors in house, but we don't know if they were all working," Smith said.

Naperville resident Avery Spunt was driving nearby at the time of the blaze and followed sirens to the scene. A total of 34 firefighters, seven fire trucks and engines and four ambulances responded to the scene.

"It really looked like quite a small fire," he said. "Shockingly, they started pulling people out, which you don't expect to see on a Saturday afternoon. It's more like a scene you see on the news in the middle of the night."

12-year-old Jillian Petterek was driving by with her mother and sister, when she noticed smoke coming from a second-floor window and called 911. They came back Sunday to pay their respects.

"It was very upsetting, very sad," said Petterek.

"It was very nerve-wrecking when I discovered smoke," said neighbor Kathy Petterek.

Spunt said he didn't see any flames outside the house when he arrived.

"I live down the street and I didn't see anything," said Kent Moeller, who has lived in the subdivision since it was built in 1993. "I came outside, and I saw some fire trucks and police cars. I saw no smoke. I saw really nothing but a lot of commotion."

But inside the house, the fire consumed a hallway and three bedrooms on the second floor, including the bedroom where the victims were found. It was estimated $100,000 worth of damage was done to the house.

"All three of them looked really terrible when (officials) brought them out," Spunt said.

"I wave to every little guy or girl I see, but - God almighty - I never dreamed anything like this would happen," Moeller said.

The Fire Department would not comment on the cause of fire. It is currently being investigated by the joint investigation team of the Naperville Fire and Police departments.

No emergency personnel were injured as a result of the firefighting and rescue operations.

Richard Fick has lived four houses from the scene since the subdivision was built. He wasn't home when the fire occurred but saw the fire trucks outside when he returned.

"I don't think anything like that has ever happened around here," he said. "We'll pray for them, certainly."

Investigators expect to have more information on the case Monday.

(CBS 2, the Naperville Sun and the Aurora Beacon-News are news partners covering stories in the western suburbs of Chicago.)