APPLETON - Police provided additional details Friday about the gunfire that killed two people and injured two others Wednesday at the Valley Transit Center, but many questions remain unanswered.

Police, firefighters and paramedics responded to the transit center to provide aid to Ruben Houston III, a 47-year-old Wausau man who was arriving on a bus from outside the area and reportedly was having a seizure.

Medical care was provided to Houston on the bus. He subsequently left the bus and was encouraged to seek additional medical care. Houston started walking toward the nearby Appleton Public Library when "the incident escalated into shots being fired," according to a statement by Police Chief Todd Thomas.

"At some point (Houston) displayed a handgun, and shots were exchanged between him and our officers," Thomas said at a press conference Thursday.

Two Appleton police officers — Sgt. Christopher Biese and Paul Christensen — fired "in response to the threat," Thomas said Friday.

In the exchange of gunfire, Houston and Appleton firefighter Mitchell F. Lundgaard were shot and killed. Houston died at the scene; Lundgaard died at a hospital.

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Christensen and bystander Brittany L. Schowalter, 30, also were shot. Christensen was released from a hospital Thursday. Schowalter is recovering from her injuries at a local hospital.

The Green Bay Police Department is investigating the shooting, and its findings will be reviewed by the Outagamie County District Attorney's Office. Appleton and Green Bay police have an agreement to investigate each other's deaths when officers are involved.

State law requires an outside agency to investigate when police kill someone.

Biese and Christensen were placed on administrative leave, which is protocol while a shooting is investigated.

Among the questions yet to be answered:

What was Houston's medical emergency?

How did Houston recover sufficiently to engage in a gunfight with police?

What caused the situation to escalate into gunfire?

Who shot whom?

Who fired first?

How many shots were fired? Thomas said Friday that Biese and Christensen fired "at least one round each." A nearby store manager told USA TODAY NETWORK-Wisconsin that he heard 15 to 20 shots.

What was captured on body camera video, squad car video or surveillance video? All Appleton police officers are equipped with body cameras.

Thomas cautioned that the information he provided was preliminary.

"As the investigation continues, we will learn more details about the incident," he said. "We know it's important to provide as much information to the community as possible, but that cannot be at the expense of compromising the integrity of the investigation."

Mayor Tim Hanna asked the community for patience while the investigation proceeds.

"There will be a time when we will talk in greater detail about what happened, about our response, about how we go forward, how we learn from this tragic incident," Hanna said.

Lundgaard, 36, was a 14-year veteran of the Appleton Fire Department. Funeral arrangements include a visitation that will be held from 3 to 5:30 p.m. Monday at Appleton Alliance Church, 2693 Grand Chute Blvd., in Grand Chute. The funeral service will begin at 6 p.m. and is not open to the public.

Gov. Tony Evers on Friday ordered flags be flown at half-staff as a mark of respect for Lundgaard. The order is in effect until sunset on the day of interment.

Contact Duke Behnke at 920-993-7176 or dbehnke@gannett.com. Follow him on Twitter at @DukeBehnke.