“It was pretty bad,” Mr. Brent said. “I was surprised that she was able to walk.”

He said he then saw officers carry two more women from the salon and put them on stretchers, he said.

Four women — between 22 and 40 years old — were treated for gunshot wounds at Froedtert Hospital, officials at the hospital said. Several had undergone surgery or were expected to soon, the officials said.

As the authorities carried victims away, Police Chief Daniel K. Tushaus said, they faced another problem: they were uncertain where the gunman was, and came upon something that initially appeared to be an improvised explosive device inside the spa — presumably left by the gunman.

The possibility that the gunman might still be loose set off new chaos, leading the authorities at the hospital where victims were being treated to put the entire facility on lockdown, preventing routine visitors from even entering the building. For hours, highway exits near the spa were closed down, some stores in the nearby mall were shut, and police officers from around the region all but filled the area.

In another Milwaukee suburb, Brown Deer, where Mr. Haughton lived, the police cordoned off a section of his neighborhood, sending residents from their homes, and checked his home with bomb-detection equipment. Neighbors said they had watched the police use a battering ram to burst through the front door and garage of his home, after shouting instructions for him to emerge. No explosives were found.