Warriors coach Steve Kerr, speaking about the latest mass shooting in Florida, doesn't believe the government is doing enough to combat gun violence. (0:28)

Chicago Cubs first baseman Anthony Rizzo, an alumnus of the South Florida high school where officials say at least 17 people have died in a shooting attack, sent his condolences and encouragement Wednesday on Twitter while sounding a note of alarm.

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."

At Stoneman Douglas, Rizzo played freshman football under assistant coach Aaron Feis, who was among those killed in Wednesday's shooting and is being hailed a hero for shielding a student.

The 28-year-old Rizzo was selected out of Stoneman Douglas by the Boston Red Sox in the sixth round of the 2007 draft. Rizzo flew from the Cubs' spring training camp in Arizona to his home in South Florida late Wednesday.

Parkland and Coral Springs please stay strong! This is out of control and and our country is in desperate need for change. I hope In this darkest of times back home this brings everyone together and we can find love. You're all in my prayers 🙏🏻🙏🏻 — Anthony Rizzo (@ARizzo44) February 14, 2018

"I told him to get back to us if there was anything we can do to help," Cubs manager Joe Maddon said Thursday. "I definitely want him to go back there and be involved. I told him, 'Take as much time as you want.'"

Said Rizzo's teammate Kris Bryant: "I know Anthony will have a big influence in Florida, but it's so sad that it happened. I can't imagine what some of those people are going through. It speaks to who Anthony really is. Anthony is a role model for everyone on this team and in Chicago and this country. I know he'll do anything he can do to help."

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."

Miami Heat guard Dwyane Wade and coach Erik Spoelstra also discussed the shooting prior to a 104-102 loss to the Philadelphia 76ers on Wednesday night.

"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.

Jaime Guttenberg, who was killed in Wednesday's shooting according to a Facebook post by her father, was a relative of Rizzo's longtime agent.

Matthew Caldwell, president and CEO of the NHL's Florida Panthers, said in a statement that 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."

Philadelphia Flyers defenseman Shayne Gostisbehere, who attended Marjory Stoneman Douglas High School, released a statement Thursday morning.

"As a former student of Marjory Stoneman Douglas High School, I am extremely saddened by the horrific news from yesterday and with a heavy heart, my thoughts and prayers go out to everyone affected by the tragedy in Parkland, Florida," read Gostisbehere's statement.

Information from ESPN's Jesse Rogers and The Associated Press was used in this report.