By Mary Wisniewski

WASHINGTON, Illinois (Reuters) - Rescue workers in a small Illinois city raked by a powerful tornado were combing through the wreckage on Tuesday in the wake of a fast-moving storm that killed eight people in two states and may have caused $1 billion in property damage.

The storm system triggered multiple tornadoes on Sunday that tore through the Midwestern United States, killing at least six people in Illinois and two people in Michigan.

"We are focusing on combing through the wreckage and seeing the damage," said Patti Thompson, a spokeswoman for Illinois emergency management agency. "We are also looking forward to the debris removal, working with communities trying to get the cleanup started."

Early damage estimates suggest that the property damage caused by the storm could reach $1 billion, with the greatest toll in Illinois, Indiana, Kentucky and Missouri, according to Risk Management Solutions, a Newark, California-based company that specializes in assessing the toll of storms and other disasters.

"Sunday's big tornado outbreak is yet another atypical storm of what has been an unusual 2013 severe weather season," said Matthew Nielsen, a meteorologist with RMS. He noted that prior to Sunday's outbreak, the United States had seen its fewest tornadoes since 1988.

Illinois Governor Pat Quinn on Tuesday added six counties to the state's declared disaster areas, expanding the total to 13. Quinn said hundreds of homes and businesses were damaged or destroyed, hundreds of thousands of people were without power and numerous roads throughout the state have been closed by fallen trees and downed power lines.

Of the six people killed in Illinois, authorities said one died in Washington, a city of 15,000 roughly 145 miles southwest of Chicago, where as many as 500 homes had been damaged in the winds of 166 to 200 miles per hour (267-322 km per hour).

Attention turned on Tuesday to assessing the damage wrought by the twister.

"When you're fortunate, you do all you can," 52-year-old Cay Ernst said while helping a friend whose house had been badly damaged look for a prized ring on Monday afternoon.

In the destroyed area, where buildings were reduced to rubble and cars turned upside down, authorities barred vehicle traffic out of concern that people could be injured while attempting to retrieve their possessions.

In central Michigan, rescue workers found the body of a 59-year-old man entangled in downed power lines on Sunday night. A 21-year-old man was killed that night when a tree fell on his car in the central Michigan town of Leslie.

(Additional reporting by Scott Malone; Writing by Eric M. Johnson; Editing by W Simon and Maureen Bavdek)