WE Energies spokeswoman Amy Jahns said workers for a contractor apparently punctured a 4-inch natural gas main, sending gas leaking into a building ahead of the explosion. All 12 gas lines in the area were shut off by about 9:30 p.m.
The blast about half a block from City Hall appeared to be centered on the Barr House, a pub. Authorities evacuated a half-mile radius and set up a shelter at Sun Prairie High School.
Steve Owen, 60, who owns Sun City Cyclery and Skates in downtown Sun Prairie, said he saw firefighters and police officers on the street and then the explosion happened. He said the building across from his shop "literally lifted up."
He said the force of the blast knocked him back in his chair and that he ran outside and saw a ball of fire.
"People were scrambling," said Owen, who lives above his shop.
Jill Thompson, 56, who lives about two blocks from where the blast occurred, said, "It shook the whole building. I thought someone had hit the building with their vehicle."