Despite several family members including the father of a missing Washington state toddler voluntarily taking a polygraph exam, police say they are puzzled as to why the boy’s mother still refuses to take the test.

Sky Metalwala, 2, was reported missing Sunday morning by his mother after she discovered him missing from her car in Bellevue, Wash.

The child's mother, Julia Biryukova, told police that she had run out of gas and left the boy buckled into his car seat, sleeping, while she and her 4-year-old daughter walked to a Chevron gas station, about a mile away. 

When the pair returned to the silver Acura about an hour later, the boy was gone, according to Biryukova.

But investigators have found that there was enough gas in the car to run a considerable distance, Bellevue police Maj. Mike Johnson said Wednesday afternoon. Police haven't ruled out some other mechanical problem with the car.

Police also said they never found a gas can and said there was no sign of forced entry inside the car. Investigators have since collected potential evidence from Biryukova's apartment as well as surrounding trash bins.  

"We suspect foul play," Mike Johnson of the Bellevue Police Department told Fox affiliate KCPQ-TV.

Johnson says he believes that Biryukova or someone close to her knows what happened to the boy. Her attorney has not returned calls seeking comment.

"Nothing about the story adds up. Something else happened and we're trying to figure out what that is," Johnson said. "Someone close to this family knows what happened to this child and we need that answer."

Biryukova and her husband, Solomon Metalwala, have been involved in a heated divorce battle. 

In court documents, Metalwala said his wife suffered from mental illness and had dreams about killing the couple's children, according to the station.

Biryukova has accused Metalwala of domestic violence, claiming he assaulted her in front of the children in December 2009. 

Biryukova had previously been cited for leaving a child unattended in a vehicle. In 2009, Redmond police cited Biryukova and Metalwala for leaving a child unattended in a vehicle. The case was dismissed after they agreed to probation, community service and a 10-week parenting class.

Police have not named either parent as suspects or "persons of interest." 

The toddler is of Indian and Caucasian descent, with brown eyes and dark hair, according to police. He was reportedly last seen wearing a dark green hooded sweatshirt and blue and gray-striped pants.

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The Associated Press contributed to this report.