"We have to be clear about this," Koval said. "He was unarmed. That's going to make this all the more complicated for the investigators, the public, to accept, to understand ... why deadly force had to be used."
The officer who killed Robinson, 12-year department veteran Matt Kenny, had used deadly force before, shooting and killing a man in 2007, the chief said.
Kenny was exonerated of any wrongdoing and even received a commendation, the chief said, adding that the incident was "concluded to be a suicide by cop" situation.
Robinson was shot after allegedly assaulting Kenny, who was responding Friday evening to a reported disturbance at a Madison residence, Koval said.
Afterward, protesters in this university town took to the streets and converged on City Hall, chanting "Black lives matter."
Koval called for calm, while acknowledging the protests are reminiscent of those that followed the deaths of black men in Ferguson, Missouri, and Staten Island, New York, after confrontations with police.
Police described Robinson as African-American.
"In light of so much things that have happened not just across the country, but in our own community, it's understandable that the reaction at the scene and of some of our citizens is extremely volatile, emotional and upsetting," the police chief told CNN affiliate WKOW-TV
. "And we understand that. That's absolutely appropriate under these circumstances. We would urge, obviously, that everyone exercise restraint."
Koval said he visited the home of Robinson's mother late Friday night and met Robinson's grandparents outside the house.
They talked and prayed, the chief said, but they advised him to put off visiting Robinson's mother "based on the dynamics" of the situation.
"I couldn't even begin to get my hands around the enormity of the loss and the tragic consequences," he said. "Nineteen years old is too young."
Robinson's mother, Andrea Irwin, told CNN affiliate WKOW that she didn't understand what happened. "My son has never been a violent person, never," she said. "To die in such a violent way baffles me."
Irwin said Robinson served as a father figure to her other children.
"He was our caretaker and so gentle," she said.
Chief: Young man 'assaulted my officer'
The incident started when authorities got a call that a black male was yelling and jumping in front of cars, Koval said. Dispatchers identified him as Robinson, according to 911 audio obtained by WKOW.
A little later the dispatcher says, "Apparently Tony hit one of his friends, um no weapons seen."
About four minutes later, the dispatcher says, "I got another call for the same suspect at [the same address]. He tried to strangle another patron."
About 30 seconds later, an unidentified officer says, "Shots fired, shots fired."
When Kenny went to the apartment, he heard some commotion and forced his way in, Koval said.
"Once inside the home the subject involved in this incident -- the same one allegedly out in traffic and that had battered someone -- assaulted my officer," Koval said.
After that, according to the chief, "The officer did draw his revolver and subsequently shot the subject."
Backup officers and others at the scene performed CPR on the young man, who later died at the hospital.
Kenny suffered a blow to the head, but is being treated and will be released, Koval said. Kenny has been placed on administrative leave with pay.
Koval said he's not sure what Robinson was doing at the house in the first place.
"His relationship to the home is unclear to me, although there were certainly familiar acquaintances. This was not a random place. He had hung out."
Mayor: Incident 'an enormous tragedy'
In a statement Saturday, state Attorney General Brad Schimel said he "can only imagine the heartbreak" of Robinson's parents and added he's "concerned for the officer ... who, I imagine, is experiencing great trauma as well."
"They are all in my thoughts and prayers," Schimel said.
Under Wisconsin law, officer-involved shootings are investigated by an outside agency, in this case the Division of Criminal Investigation. Once DCI completes the investigation, the report will go to the local district attorney, Koval said.
Some are demanding answers sooner rather than later.
On Friday night, dozens of demonstrators came out to the area around the apartment, which police had blocked off. A group also moved toward City Hall before dispersing early Saturday.
"Who do we trust?" some called out, prompting the response, "No one!"
And in another refrain, they chanted, "Black lives matter."
The protesters' sentiments were echoed online, where some adopted the #WillyStreet hashtag in reference to Williamson Street, where the shooting happened.
"Praying for Madison tonight," wrote one activist. "Stand up, sit in, walk out - until u get answers. And until there are no more hashtag eulogies."
Mayor Paul Soglin spoke to the raw feelings, calling what happened "an enormous tragedy."
"We've got a family that's really hurting," Soglin said, according to WKOW. "And we've got a city and neighborhood that's feeling pretty well hurt itself."
Some of the protesters were members of the Young, Gifted and Black Coalition that was formed last summer after the Michael Brown shooting in Ferguson.
The group wants more reactive policing in African-American neighborhoods, said group member Brandi Grayson.
She said Madison police park on street corners in African-American neighborhoods and wait for something to happen, which leads to residents being hassled. That doesn't happen in white neighborhoods, she said.
She said Young, Gifted and Black will be working with church groups on Sunday and will hold more rallies Tuesday and Wednesday.