"The ground shook," one unidentified witness told CBS radio station WBBM-AM. "I saw a huge billowing of smoke...moments after that, a huge fireball. I mean, it was unbelievable to watch this."
Witnesses said they heard two large explosions at 7:50 a.m. local time at a coal-fired Southern Energy Inc. power plant along the Lake Michigan shore at the Illinois-Indiana state line. Thick black smoke and flames were still billowing from the plant hours later.
Of the 16 people injured in the blast, 10 people were injured seriously enough to require hospitalization. Some of those injured had to be taken by helicopter to area hospitals.
Three people had remained trapped on the roof of the burning building, but they have now been rescued, the Associated Press is reporting.
All plant employees have been accounted for, but plant officials continue to check on contractors and visitors who may have been on the site.
Southern Energy purchased the plant from Chicago-based Commonwealth Edison Co. in December. Jann Benson, a spokeswoman for Southern Energy parent Southern Co., said officials were still trying to determine the cause of the explosions but were more concerned about the safety of the workers.
"That's all we're thinking about right now is their safety," Benson said.
There were outside cleaning contractors at the plant when the explosion occurred, said Hammond fire inspector Jim Walsko.
The plant delivers 490 megawatts of capacity to ComEd under the terms of a 15-year power purchase agreement, according to information on Southern Energy's Internet site.
The boiler is designed to produce 3.5 million pounds of steam per hour, which is used to provide energy to turbine and in-plant systems.
There were several reports of short power surges in Chicago about the time of the explosions.