TORONTO - Fire officials continued Wednesday to probe "suspicious" elements of a fire in a Toronto highrise that killed a pregnant woman and her young son and left two other people in critical condition.

Investigators were concerned by witness reports that one of the occupants of the burning suite came to the door during Tuesday's fire but didn't make it out.

"There are definitely aspects of this fire that cause us to be suspicious," said Wayne Romaine of the Office of the Ontario Fire Marshal.

Police said Wednesday the cause of the fire had not been determined.

Four victims, including a four-year-old boy, were pulled without vital signs from a bedroom in the ninth-floor suite while flames raged in the front of the apartment, where the fire broke out.

Two women suffering from smoke inhalation remained in hospital in critical condition Wednesday.

Investigators have not clarified the relationship between the hospitalized women and the victims.

Police also declined to release the names of the victims, saying they were awaiting the results of Thursday's post-mortems and ensuring next of kin were notified.

A distraught woman who identified herself as the victim's sister showed up at the apartment in northwest Toronto clutching pictures of the young woman and her son.

"I talked to her every single day. This is us at a birthday party. Look at us, we're always together," the woman said through sobs, showing the photos to reporters.

"She was my best friend."

On Wednesday, tenants at the highrise said fire alarms are frequent in the building but often turn out to be false. Roger Smith wondered whether that proved to be a fatal mistake.

"People are always pulling the fire alarm," he said. "I guess no one took it seriously."

One longtime tenant said she was surprised when the alarm went off and the smell of smoke followed.

"I was amazed that there was actually a fire," said the woman, who declined to give her name.

Toronto police Det. Andrew Stinson said the apartment where the fire broke out was being treated as a "scene under investigation," and downplayed earlier indications from police that investigators were treating the suite as a crime scene.

"After the autopsies, depending on the results of those, then we'll work with the fire marshals on determining which way the investigation will go," Stinson said.

Smoke was so thick in the unit that firefighters were unable to see and had to feel their way around, but crews managed to get the blaze under control within 15 minutes.

Toronto fire Chief William Stewart called it "unusual" and "alarming" that four people were found without vital signs at 6 p.m., a time of day when most people would normally be awake and alert.