"While I don't have all the answers, I know that something has to change, before this is visited on another community, and another community, and another community," Rizzo, a 2007 graduate of the school, said Thursday night at a candlelight vigil for the victims of Wednesday's massacre.
Seventeen people, including students and school workers, were killed when a gunman opened fire with a semiautomatic rifle at the school in South Florida.
Rizzo left Cubs training camp in Arizona on Wednesday and was one of about two dozen speakers who addressed a crowd of thousands who came out to show their support at the Parkland Amphitheater.
"I come home to Parkland to what should be everybody's first concern, and that's showing our kids out there -- the students at Stoneman Douglas and of Broward County and from all over the country -- that we care about their lives and about their future," Rizzo said. "I've been very impressed with talking to the students and how they're taking care of each other and how they're coming together. I'm so grateful to the teachers, the coaches, the administration and all the first responders that tried to protect them."
Wearing a black polo shirt with a red ribbon pinned above his right chest, Rizzo was seated on the amphitheater stage along with spiritual leaders, government officials and family members of the deceased.
Rizzo took part in a moving candle-lighting ceremony as the names of the 17 victims were read aloud. Shortly after that, he took the podium and spoke for four minutes, pledging his support to an emotional crowd that included his mother, father and fiancée.
"I am only who I am because of this community," Rizzo said. "And I just want all of you to know how proud I am to be a part of this community. I want you to know that you're not alone in your grief. We're all grieving with you. The entire country is grieving with you. So whatever comfort I can give, I will give. Whatever support I can offer to our students, teachers, coaches and families and first responders, you'll have it."
Rizzo showed his support during Thursday's hourlong ceremony by rising to his feet several times to join thunderous ovations as Broward County Sheriff Scott Israel and others made impassioned pleas for "common sense gun laws."
"I promise you we're going to be mourning, grieving and a bit broken for a while," Rizzo said during his speech. "We're human. But I promise the cameras are going to move on. The demands of everyday life will intrude again. Classes will start again. The seasons are gonna change, and the sun is going to rise. And all we'll have left is each other.
"We don't know who's hiding their sadness or feelings of guilt and loneliness, or who needs help and is too proud or afraid to ask. So we have to be there for each other, we have to cope with our pain, and we have to live each other's pain. We have to be the best possible versions of ourselves."
After the vigil concluded, Rizzo declined to speak with reporters. It's unknown how long he'll remain in Parkland before rejoining the Cubs in Arizona.
Manager Joe Maddon exchanged text messages Wednesday night with Rizzo in the aftermath of the nation's deadliest school attack in five years. He said the team was "all for" Rizzo leaving spring training to offer support in Parkland.
"Told him to get back to us if there's anything that we can do to help," Maddon said earlier Thursday. "I definitely want him to go back there and become involved, as he should. It's just horrible. ... What are the proper words right now? I don't even know what the proper words are except that 'we're there for you.'"
Cubs outfielder Albert Almora Jr., who is from outside Miami, recalled Rizzo hitting at his old high school a couple of times last season, noting, "I'm sure he still has a lot of connections there."
Before starting drills, the Cubs huddled together on the grass under cloudy skies following more morning rain.
"It's an awful situation," Almora said. "The only thing we can do is come together. Obviously we're all here for Anthony and for that school and all those families."
Maddon called Rizzo "the rock on the field" for the Cubs.
"Please go. Please take your time," was the manager's message to Rizzo. "Please do what you think you need to do and let us know is there some way that we can become involved and help."
The Associated Press contributed to this report.