MORGANTON, N.C. - An explosion at a chemical plant Tuesday injured at least 16 workers and set off an intense blaze spewing thick, black smoke that forced nearby residents indoors while crews tried to determine if it was harmful to breathe. Windows were blown out up to a third of a mile away.

Officials don't know what caused the explosion, said Lisa Propst, a communications manager for Burke County emergency services.

Authorities called residents near the plant to tell them to close their windows and turn off their ventilation and heating systems while hazardous materials crews figure out what chemicals were involved in the blast, she said.

The blast shattered windows at Pritchard Siding Co., a third of a mile from the Synthron plant, said owner Lola Pritchard.

"Right now, I'm in the middle of cleaning up glass and stuff," she said. "It blew our windows out."

Two victims were airlifted to different hospitals after the 11:30 a.m. explosion at the Synthron Inc. plant, a hospital spokesman said. One was in serious condition and the other's condition was not released.

Fourteen people, none with life-threatening injuries, were being treated at another facility, officials said.

Synthron is a subsidiary of Paris-based Protex International, which produces specialty chemicals in the United States, Europe, Asia and North Africa. There was no immediate response to calls seeking comment from Protex headquarters in Paris after business hours.

Four state Labor Department safety and health inspectors were headed to the scene, agency spokeswoman Heather Crews said. Also en route was a water quality specialist from the state Department of Environment and Natural Resources.

State Labor Department records showed that inspectors last found problems at the plant in 1996, when the company was fined $602 for 10 violations, five of which were classified as serious.