Soloist Olivia Knutsen delivered a stirring rendition of the national anthem Tuesday at Ground Zero in New York City, as the United States marks 17 years since the 9/11 attacks. 

The National September 11 Memorial & Museum held a ceremony at Ground Zero in Lower Manhattan to remember the nearly 3,000 people killed in the attacks on the World Trade Center, the Pentagon and aboard Flight 93, which went down in Shanksville, Pennsylvania.

Knutsen sang "The Star-Spangled Banner" to kick off the ceremony, with New York City Mayor Bill de Blasio, New York Gov. Andrew Cuomo, Ambassador to the United Nations Nikki Haley and former New York City Mayor Rudy Giuliani in attendance, along with survivors of the attack and hundreds of 9/11 victims' families. 

President Donald Trump and First Lady Melania Trump are marking the solemn anniversary by participating in a ceremony at the 9/11 memorial in Shanksville, and Vice President Mike Pence spoke at the Pentagon.

Giuliani, who was mayor at the time of the attacks, joined the "Fox & Friends" co-hosts to reflect on the solemn anniversary.

"First couple of days were just gruesome, the realization that we lost that many people, the realization that we weren’t going to discover bodies," Giuliani said. "That was a shock. That was an absolute shock to me."

He said that as the scope of the tragedy came into focus, he was moved by the outpouring of support from people across the United States and world.

"It happened with firefighters from Chicago and Philadelphia and Florida and Texas. It happened with school classes all over the country. It happened with a bunch of Canadians -- 25,000 of them coming to New York in November," Giuliani said. "So, that got you through, That said to you, 'Oh my gosh, we have a whole country behind us, if not a whole lot of decent people throughout the world.'"