Rizzo, who in November donated $150,000 to his alma mater, Marjory Stoneman Douglas High School, for the installation of lights for baseball and softball fields, said he hoped the people of Parkland, Florida, would come together in "this darkest of times."
Rizzo, 28, was selected out of Stoneman Douglas by the? Boston Red Sox?in the sixth round of the 2007 draft.
The 19-year-old shooter, who was equipped with a gas mask and smoke grenades, set off a fire alarm to draw students out of classrooms shortly before the day ended at one of the state's largest schools, officials said.?Marjory Stoneman Douglas has about 3,000 students.
More than a dozen other people were wounded and taken to hospitals, doctors said.
Golden State Warriors coach Steve Kerr, known for speaking out on gun control and other social issues, weighed in on the shooting prior to Wednesday night's game at Portland.
"It doesn't seem to matter to our government that children are being shot to death day after day in schools; it doesn't matter that people are being shot at a concert, at a movie theater," said Kerr, whose father was shot and killed in January 1984 while president of the American University in Beirut. "It's not enough apparently to move our leadership, our government, the people who are running this country, to actually do anything. That's demoralizing."
"It's very unfortunate, all the families dealing with the unknown of what's going on with their kids, our prayers go out to them," Wade said. "The families that have been affected by this incident, our prayers go out to them, it's so unfortunate."
Added Spoelstra: "It's horrifying as we're finding more information about it, I just want them to know we're thinking about them. It makes this game feel totally insignificant. And it's just mind-boggling."
The? University of Florida?held a moment of silence Wednesday night prior to its basketball game in Gainesville against Georgia.
The Broward County sheriff said the suspect, identified as Nikolas Cruz, was in custody and investigators were starting to investigate the attack at the school.?
The suspect, a former student, was previously expelled for disciplinary reasons, Sheriff Scott Israel said.
Most of the fatalities were inside the building, though some were found fatally shot outside.
It is the nation's deadliest school shooting since a gunman attacked an elementary school in Newtown, Connecticut in 2012.
Matthew Caldwell, president and CEO of the NHL's? Florida Panthers, said in a statement the organization is committed to "providing aid and support to our community as it grieves in response to this senseless tragedy."
After the team's 4-3 win in Vancouver on Wednesday night, coach?Bob Boughner said "we talked about the devastation and hurt back home before the game. We wanted to represent our community with a big effort tonight, but we were playing with heavy hearts."
Information from The Associated Press was used in this report.