Chief Ziman said officers from other departments converged on the site, and teams were assembled to search for the gunman inside the sprawling, 29,000-square-foot building. When they eventually found him, about 90 minutes after the search began, they shot and killed him, she said. The police said he was armed with a handgun.

John Probst, an employee who was in the building at the time, told ABC 7 Chicago that he recognized the gunman, a co-worker, “running down the aisle” with a pistol that had a green laser attached to it.

He said he saw one person who was shot and “was bleeding pretty bad.” He believed there were others who had been shot in the office. He said he and another co-worker ran out the back door, and he heard more shots once he was outside.

Officials from Mueller Water Products, of which the Henry Pratt Company is a subsidiary, issued a statement late Friday expressing shock and sadness.

“Our hearts are with the victims and their loved ones, the first responders, the Aurora community and the entire Mueller family during this extremely difficult time,” the statement said. “Our entire focus is on the health and well-being of our colleagues, and we are committed to providing any and all support to them and their families.”