NAPERVILLE, Ill. – A suburban Chicago mother bought a can of gasoline, shut herself in her bedroom with her two young children and set a blaze over the weekend that ended up killing all three of them, police said Monday.

“It is tragic and it is senseless. I can’t explain why. I can’t give you the reason,” Naperville Police Chief David Dial said.

Investigators believe 32-year-old Nimisha Tiwari – who police said was in a “troubled marriage” – set herself on fire, and rescuers found all three ablaze on a bed in the master bedroom, Dial said. Also killed were Tiwari’s 4-year-old son, Vakadham, and 18-month-old daughter, Anaya.

Police have video of the woman with the children in the family van buying a can of gasoline a few hours before the Saturday afternoon fire. She then took them to a store and bought toys, including a doll and a train. The gas can and toys were found in the burning bedroom, Dial said.

Rescuers were called to the home in this suburb west of Chicago after a passer-by spotted smoked coming from the second story. Firefighters had to force their way in because the front door was bolted shut, Dial said.

The woman’s husband, Anand Tiwari, was not home at the time of the fire and authorities have corroborated information that he was taking business classes in Chicago.

The husband asked to be given a polygraph test and “the results of that have given us no reason to disbelieve anything that he has said in connection with this matter,” Dial said.

Dial said the husband is “distraught” over the loss of his family. The couple married in 1999 in India and came to the United States shortly afterward. In May, both the husband and wife had contacted Naperville police about a “difference of opinion” in their marriage.

The same month, Nimisha Tiwari filed for an order of protection against her husband, the Chicago Tribune reported Monday, citing court records. In those documents, she claimed he abused their son, drained their joint checking account, pushed her mother and encouraged Tiwari to commit suicide.

Lawyers for Anand Tiwari later filed documents denying his wife’s allegations, the newspaper reported. The documents said she suffered from multiple sclerosis that caused “her to create false beliefs for herself” and affected her “decision-making.”

The documents also accuse the wife of punishing their son by locking him in a dark room, alleging the son “is terrified of his mother unless I am there with him.”

Dial declined to comment on a “medical issue” that he said the husband indicated had caused problems in the past.

The Tiwaris moved to the home three or four years ago, and their neighbors on Monday expressed shocked about the deaths.

“The kids were very nice,” said Steven Naday, who lived a few houses away. “The 4-year-old, really he was a smart little guy.”

Chandrakanth Shet, a next-door neighbor, said his family and the Tiwaris often spent time at each other’s homes. His 5-year-old daughter often went next door to play with Vakadham, he said.

“(The mother) was a nice person, a kind person,” Shet said. “Both parents loved their children.”