This is What’s Trending Today.
On the morning of September 11, 2001, terrorists connected to al-Qaida hijacked four passenger airplanes. They flew two of them into the World Trade Center, known as the "Twin Towers," in New York City and another into the Pentagon building in Arlington, Virginia.
The Pentagon is the headquarters of the U.S. Department of Defense, across the Potomac River from Washington, D.C.
Just a few minutes later, passengers on the fourth airplane took control from hijackers and crashed their airplane into a field in Shanksville, Pennsylvania. The plane was thought to be heading for Washington.
The attacks killed more than 3,000 people.
On Monday morning, President Donald Trump led a moment of silence at the White House and spoke at a memorial service at the Pentagon.
At the memorial service, Trump said "the terrorists who attacked us thought they could incite fear and weaken our spirit…But America cannot be weakened."
Vice President Mike Pence traveled to Shanksville in the state of Pennsylvania to attend a service there.
During the day, Twitter users wrote about September 11 using the hashtags #NeverForget and #September11. World Trade Center was also a trending topic.
Some people wrote about their own experiences that day. Others posted photos and videos of the memorials and memorial services.
A New York City police officer rang a bell where the Twin Towers once stood at 10:28 a.m. That is the time when one of the towers fell 16 years ago.
At the same time, sunlight brightened an atrium at the new World Trade Center. The building is designed to let a strip of light in at 10:28 a.m. each September 11. The feature is known as "The Oculus."
At the memorial, family members of the victims read their names out loud.
Sara Clarke is a journalist in Washington, D.C. She posted a photo of the memorial in Shanksville. She was in Pennsylvania on September 11 last year.
Recent U.S. Presidents Barack Obama, George W. Bush and Bill Clinton all used Twitter to recognize the anniversary of the attacks.
Bush called for remembering "lives stolen," Clinton called the rescue workers who helped save many lives that day "heroic" and Obama wrote: "No act of terror will ever change who we are."
And that’s What’s Trending Today.
I’m Dan Friedell.