Front-runner Donald Trump tweeted: "Let's all take a moment to remember all of the heroes from a very tragic day that we cannot let happen again!"
At a service on the 1th anniversary of the attacks, Bush said, "I'm proud of meeting the families of firefighters. To this day, when I go to New York, I always stop at a fire station to say thanks for their incredible sacrifice and the services of the emergency responders."
The former Florida governor spoke at a remembrance in Londonderry, New Hampshire, which started at 8:46 a.m., marking the time the first plane hit the north tower of the World Trade Center. He later began reading the names of the victims in a ceremony that was expected to last until noon.
He added, "I'm proud of the president of the United States at the time who unified our country in a way that was desperately needed and created a strategy to keep us safe."
At a town hall meeting in New Hampshire Thursday night, Bush recounted where he was just one day before the attacks, having dinner with his brother George and a friend. The sense of normalcy before the attacks, he recalled, was "surreal."
"We had dinner with friends of mine, and supporters of his, and spent two hours talking about books we were reading, it seems so surreal now," he said.
New Jersey Gov. Chris Christie recalled on "Fox and Friends" how then-President Bush had called him the day before Sept. 11 to let him know he was appointing him federal prosecutor. Christie took the next day off, brought his kids to school, and then his wife called him from Manhattan to tell him a plane had struck the World Trade Center.
"While we were on the phone talking about other things, the second plane hit the second building," Christie said.
He then lost track of his wife for five hours before she reached a working pay phone in Manhattan. The next time he saw her was when she walked off the ferry from Manhattan.
"When she got off that ferry, she was walking down the street and she was wrapped in a blanket and soaking wet -- anybody who had been in lower Manhattan, they were hosed off by firehoses. They were covered with dust," Christie said. "That's' the first sight of my wife after having left her that morning. I just grabbed her, hugged her, got her in the car and got her home."
Bush later tweeted Friday that people should remember another Sept. 11 attack, the one on the U.S. diplomatic facility in Libya "On 9/11, we should also pause to remember 4 Americans killed by terrorists in Benghazi, Libya on this day in 2012. They are not forgotten."
Former New York Gov. George Pataki recounted his experience that day on MSNBC's "Morning Joe" -- from being alerted by his daughter when the first plane hit, to going against his security's recommendations that he leave Manhattan for Albany.
The smoke and fire at Ground Zero turned lower Manhattan into a "Dante's Inferno," he said.
"I went down and actually that night was at Ground Zero, and the flames were everywhere. You could taste the air from one river to the other. If ever there was Dante's Inferno live, it was lower Manhattan that day. And yet I remember, too, and this is another terrible memory, on January 1st, almost four months later when Mike Bloomberg was sworn in at Gracie Mansion, you could see the smoke still rising from ground zero a few blocks to the south," Pataki said. "Almost four months later it was still burning."
Other 2016 hopefuls weighed in throughout the day Friday. Wisconsin Gov. Scott Walker, who was a state lawmaker at the time in 2001, released a statement praising the first responders and cautioning against radical Islam.
"On that terrible day, radical Islamic terrorists unleashed unimaginable horror against our country, ending nearly 3,000 innocent lives. As we honor the victims and keep their families in our thoughts and prayers, we cannot forget the bravery of the hundreds of first responders who sacrificed their lives while trying to save others," Walker said in a statement.
Florida Sen. Marco Rubio,sent his thoughts in a string of tweets Friday morning.
"Today my thoughts and prayers are with all those affected by the terrible attacks 14 years ago. America will not, and must not, ever forget," Rubio tweeted.
Former Maryland Gov. Martin O'Malley, who was Mayor of Baltimore at the time of the attacks, tweeted "On this day in 2001, we mourned as a city, a state, and a nation. Today, we remember those who lost their lives in the attacks."