CHICAGO (CBS) — The 3-year-old boy who was killed in an extra-alarm fire in the Grand Crossing neighborhood on Sunday had been left alone in a locked apartment.

Maqkwone Jones was in the care of his foster mother when the fire started around 10:30 a.m. Sunday in a three-story apartment building near 67th Place and Dorchester.

The foster mother was not home at the time, and a friend who was sleeping in the apartment when the smoke alarm went off was not aware Maqkwone was still there, according to the Fire Department and Illinois Department of Children and Family Services.

That friend ran out of the building when the fire started, and called the foster mother, who asked if she had taken the child with her, according to DCFS. When the friend told firefighters the boy was still inside, they went into the building and brought him outside.

By then, Maqkwone had apparently suffocated from smoke inhalation, according to DCFS.

He was pronounced dead at 11:36 a.m. Sunday at Comer Children’s Hospital. An autopsy on Monday was inconclusive, according to the Cook County Medical Examiner’s office, pending further investigation.

According to published reports, the Chicago Fire Department believes the cause of the fire was suspicious, and the investigation has been turned over to the Chicago Police Department.

DCFS said Makqwone had three siblings who also are in foster care, but they were with another relative at the time of the fire. They have been moved to other foster homes while DCFS investigates Makqwone’s death.