President Trump didn't shy away from his push for Second Amendment rights during Tuesday night's State of the Union address. But as he spoke about the importance of protecting the right to bear arms, the father of a teenage girl killed in the 2018 Parkland, Florida mass shooting yelled out — and was promptly ejected.

Fred Guttenberg's 14-year-old daughter, Jaime Guttenberg, was one of 17 people killed at Marjory Stoneman Douglas High School on Valentine's Day in 2018 when former student Nikolas Cruz brought an AR-15 into the building. House Speaker Nancy Pelosi invited Guttenberg to attend the address. He's been an outspoken advocate for gun reform since the shooting.

Thank you @SpeakerPelosi for inviting me to be your guest tonight at the State Of The Union. I cannot thank you enough for your commitment to issues important to Americans and to the issue most important to me on dealing with gun violence. — Fred Guttenberg (@fred_guttenberg) February 4, 2020

Leading up to Guttenberg's outburst, the president said, "In America we don't punish prayer; we don't tear down crosses; we don't ban symbols of faith; we don't muzzle preachers and pastors. In America, we celebrate faith. ... Just as we believe in the First Amendment, we also believe in another constitutional right that is under siege all across the country. So long as I am president, I will always protect your Second Amendment right to keep and bear arms."

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Through the applause that followed — mostly from Republicans — and images of Democrats looking angry and remaining seated, viewers could hear Guttenberg yell out. Pelosi was seen looking concerned, in Guttenberg's direction, as Mr. Trump continued to speak.

Fred Guttenberg, father of Parkland school shooting victim Jaime Guttenberg, is ejected after shouting during President Trump's State of the Union address ON February 4, 2020. Tom Brenner / REUTERS

Huffington Post congressional reporter Matt Fuller tweeted that Guttenberg yelled something along the lines of "victims of gun violence like my daughter." CBS News hasn't confirmed what Guttenberg said. He was promptly kicked out.

A few hours after the address, Guttenberg tweeted an apology for his outburst, saying, "I let my emotions get the best of me. I simply want to be able to deal with the reality of gun violence and not have to listen to the lies about the 2A [Second Amendment] as happened tonight."

Guttenberg said he shouldn't have yelled out, though he is "thankful for the overwhelming support" after he did.

"I do owe my family and friends an apology," he wrote. "I have tried to conduct myself with dignity throughout this process and I will do better as I pursue gun safety."

(1,2) Tonight was a rough night. I disrupted the State Of The Union and was detained because I let my emotions get the best of me. I simply want to be able to deal with the reality of gun violence and not have to listen to the lies about the 2A as happened tonight. — Fred Guttenberg (@fred_guttenberg) February 5, 2020

On Monday, Guttenberg tweeted in memory of his daughter, who was a freshman at Marjory Stoneman at the time of her death, for National Gun Violence Survivors Week. He wrote about watching her grow up and how he "used to dream of our future together and watching her graduate, turning 16 and driving, walking her down the isle [sic] and becoming a grandparent to her children."

"Our relationship will live on forever, however, it will always be different," he wrote.

(1,5) This is National Gun Violence Survivors week and in honor of that I want to talk about my daughter Jaime. Our relationship has changed dramatically over the past two years since February 14, 2018. I used to dance with Jaime, laugh with Jaime, watch TV with Jaime, pic.twitter.com/TVDiDQGRtx — Fred Guttenberg (@fred_guttenberg) February 3, 2020

Shortly after Guttenberg was escorted out of Pelosi's box, Twitter users started posting their support, with the hashtag "#ImWithFred."

David Hogg, a former Marjorie Stoneman student who became a founder of March for Our Lives, tweeted that Guttenberg "stood up to a president that believes peace and the Second Amendment are mutually exclusive."

.@fred_guttenberg lost his daughter in the Parkland shooting and has worked every day since to end gun violence. Tonight he stood up to a president that believes peace and the second amendment are mutually exclusive and was removed.#ImWithFred pic.twitter.com/RprctMp2iE — David Hogg (@davidhogg111) February 5, 2020

That protestor is @fred_guttenberg, who's daughter Jaime Guttenberg was killed in the Parkland massacre. Trump did not mention gun violence once during the #SOTU . He wants to defend the right to bear arms? What about our right to not be shot?#IStandWithFred https://t.co/JxLWphiZLK — March For Our Lives (@AMarch4OurLives) February 5, 2020

Actress and activist Alyssa Milano tweeted that Guttenberg is a "hero and an inspiration" who "won't rest until America's children are safe from gun violence."

My friend @fred_guttenberg is a hero and an inspiration. He’s honoring his daughter with action. And he won’t rest until America’s children are safe from gun violence. #ImWithFred https://t.co/mjflutf9iP — Alyssa Milano (@Alyssa_Milano) February 5, 2020

Mr. Trump spoke about the push for Second Amendment protections throughout his speech, but didn't mention the numerous mass shootings during his time as president. In 2019 alone, there were more of them than days in the year.