LINDEN, N.J. (CBSNewYork) – A 9-year-old boy is dead and his 10-year-old sister is in critical condition from carbon monoxide poisoning in Linden Tuesday morning, police said.

Their mother found them unresponsive at around 7:25 a.m. in their bedrooms at 1157 Passaic Avenue, police said.

She began CPR. Emergency responders arrived and the girl was rushed to Newark Beth Israel Medical Center in extremely critical condition, according to police. The boy was rushed to Trinitas Hospital where he was pronounced dead, police said.

Officials say the incident appears to be a tragic accident caused by improper venting of fumes from the furnace.

There was a carbon monoxide detector in the house but it was apparently not working, officials said.