One People's Gas employee has died after being struck by a pipe while pressure-testing it beneath the street near Willis Tower Wednesday afternoon.
A second utility worker remains in critical condition at Stroger Hospital.
The employees were using compressed air to test the pressure in a 20-inch gas main beneath the area of Wacker and Jackson when the pipe burst, the fire department said.
"They were pumping air through the system when something went wrong," Fire Media Affairs spokesman Quention Curtis said. "It blew the pipe apart and it hit them."
"I was taking pictures of the (Willis) Tower, and I heard a little pop, like a sandwich bag explosion or a milk carton, like when you were a little kid and you'd pop carton," said witness Todd Hinden, who described both employees as men who looked to be at least 40 years old.
There was no gas involved, the utility said.
A viewer wrote NBCChicago and reported that a bystander jumped into the hole, apparently to try to help the workers. Arriving emergency crews used a ladder "to pull a lifeless body out of the hole."
Another worker who appeared to be shaken but "largely unharmed" was loaded into a second ambulance. Information on the employees' identities has not been released.