FORT WORTH, Texas (AP) - A chemical plant on Fort Worth's North Side continues to smolder today after an explosion that sent four people to the hospital.

The afternoon explosion yesterday at the Valley Solvents and Chemicals plant sent fireballs and a dense column of black smoke high into the air before the flames subsided.

Fort Worth fire Lieutenant Kent Worley says workers reported they heard an explosion and then noticed a fire near a tractor-trailer that had just pulled into the plant to deliver chemicals.

Worley says none of the injuries were life-threatening.

Company president Bill Davis says there's some indication the ignition source may have come from the truck that was unloading. But he says the cause remains under investigation.