By M. Alex Johnson, Staff Writer, NBC News

A combination of human error and equipment failure might have caused a series of explosions that engulfed a propane plant in flames in Tavares, Fla., late Monday, injuring eight people and forcing evacuations for a half-mile around the scene, authorities said.

The explosions began as a fire spread through the Blue Rhino plant, near Orlando, about 10:30 p.m. ET, Lake County sheriff's Lt. John Herrell told reporters early Tuesday.

Plant officials say all employees of the plant are accounted for. All of those injured were in the plant when the fire started.

Tavares Fire Chief Richard Keith said Tuesday morning that sabotage is not expected. The fire marshal is on site to do a full investigation. 

Residents of the area told NBC station WESH of Orlando that flames from the explosions could be seen for several miles, but there were no reports of damage to the surrounding buildings. 

"It sounds like bombs are going off," Norma Haygood, a nearby resident, told WESH.

No deaths were immediately reported, Herrell said. Three male victims were in critical condition, the Orlando Regional Medical Center, told NBC News. 

Two were airlifted; a third was transported by ambulance.  

Authorities initially declared a one-mile evacuation zone around the plant, but they later scaled that back to a half-mile, effective until 6 a.m. ET. 

"We feel that there is no longer any danger" to the area around the plant, Herrell said.

Twenty-four or 26 people were scheduled to work the night shift at the 33,000-acre facility, according to plant managers, Herrell said. Authorities initially said 15 people were unaccounted for, but Herrell later said the company's management said that they had accounted for all of the people it knew were in the plant at the time of the initial explosion.

Some of them showed up at others location, according to Blue Rhino, Herrell said. He said he had no further information on where the others were.

The plant housed about 53,000 20-pound propane cylinders, Herrell said.

"They store the propane cylinders on different parts of the property, and as the fire spread, there were more and more explosions," he said. 

Explosions continued until 1 a.m. ET Tuesday, roughly 2½ hours after the first blasts were reported.

Don Ingram, former plant production supervisor at the plant, said his son felt one of the explosions in their home 6 miles away.

Ingram told WESH that the back area of the plant is "lined with propane tanks stacked four or five high on plastic pallets."

"I don't think you fight this fire," he said. "It's just too dangerous."

Blue Rhino is a subsidiary of Ferrellgas Partners, the second-largest distributor of propane in the U.S.

Azhar Fateh of NBC News contributed to this report.