PHOENIX -- Nine people were injured and one was unaccounted for after two explosions and a fire brought down an eight-unit apartment building.

Authorities had not determined the cause of the blasts, which occurred at 1 a.m. Wednesday, but assistant fire chief Bob Khan said the first explosion originated in the apartment of a caretaker named Dale Wayne Barry. Barry, 49, was missing, but it was unclear if he was in the rubble.

Barry had been served with an eviction notice for failing to pay rent, court records show.

Mayor Skip Rimsza said officials were conducting a criminal investigation into the explosion, but Phoenix police and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives said it was too early to determine whether a criminal investigation was warranted.

"We have to make a cause of determination of the explosion," ATF spokesman Tom Mangan said.

Dogs were expected to search the rubble once officials were sure it was cool enough.

A 35-year-old man was listed in critical condition early Thursday with trauma and burn injuries, said Maricopa Medical Center spokeswoman Sarah Harper. A 36-year-old woman was in fair condition.

Seven others were hospitalized with injuries that weren't considered life-threatening, Khan said.

The explosions occurred five minutes apart and leveled the two-story building, leaving only parts of three cinder block walls standing.

"The walls blew out and the roof just pancaked right down," fire Capt. Mark Angle said.

Nearby buildings were rocked by the blasts, which shattered 50 to 60 windows and catapulted bricks onto cars in the parking lot.

Shauntia Stuart, who lives in the apartment complex, said she was awakened by the sound of the explosion.

"All I could see was a big ball of fire," she said.

Investigators, including a team from the Bureau of Alcohol, Tobacco and Firearms, were expected to search the area after a structural engineer declared it safe.

A Southwest Gas spokesman said crews found no underground gas leaks.