AUGUSTA, Ga. -- Officials said another burn patient has died of injuries suffered in an explosion and fire at a sugar refinery, bringing the death toll to 13.

A little more than a month after the Feb. 7 blast at the Imperial Sugar plant in Port Wentworth, six other patients remained Friday in critical condition at the Joseph M. Still Burn Center at Doctors Hospital in Augusta.

Four are in serious condition.

A hospital spokeswoman said an 11th patient died on Friday.

The patient's name was not released.

Investigators have said the explosion was fueled by airborne sugar dust at the refinery, near Savannah. They have not yet determined what ignited it.