U.S. officials assert that the attacks were carried out by 19 al-Qaeda terrorists but many experts have raised questions about the official account.
On Monday night, the "Tribute in Light" commemorative public art installation will take place across the New York City night sky.
A third plane was hurled at the Pentagon, while another - United Airlines Flight 93 - smashed into a field in Pennsylvania.
Almost 3,000 people were killed.
"Come back at this time next year to hear the voices sing", John Reynolds, former Flight 93 National Memorial federal advisory commission chair told the crowd at the groundbreaking ceremony.
BETH A. KEISER/AFP/Getty ImagesRescue workers continue their search as smoke rises from the rubble of the World Trade Center.
The commissioners' resolution served primarily as a thumbs-up to organizers from the non-profit September 11th National Memorial Trail Alliance to continue its efforts to include parts of Bucks County in the trail's path.
Relatives, survivors, and rescuers will attend ceremonies at the three sites. The second plane hit the South Tower at 9:03 a.m. ET.
The somber service began at 9:45 a.m., the time that federal investigators determined passengers chose to revolt against their four al-Qaida hijackers, who ended up crashing the plane in a field 60 miles (100 kilometers) southeast of Pittsburgh.
GettyThe al-Qaeda terrorists who carried out the attacks.
Mark Wilson/Getty ImagesU.S. President George W. Bush sits at his desk in the Oval Office after addressing the nation about the terrorist attacks on NY and Washington, D.C.
London firefighters will gather Monday morning to honour their fallen comrades as the world marks the 16th anniversary of the September 11th terrorist attacks.
We will never forget, how in that smoke's silver lining stood a line of blue, a line of red, and line of white to show the world these colors don't run.
WSJ, Bloomberg Latest to Claim Bitcoin Exchange Crackdown in China
Just last week, China banned initial coin offerings ( ICOs ) to prevent fraud and illegal fundraising using virtual currencies. That news came after the announcement on Monday to ban ICO financing that I reported here.
Trump speaks with Qatar leader amid diplomatic crisis
The blockading states shut down air, maritime and land links with Qatar, and imposed economic sanctions the gas-rich emirate. Media outlets in Qatar and Saudi Arabia report that a phone call between the leaders of both countries has taken place.
Trump to go to Florida 'very soon'
Irma has forced millions of residents to leave their homes in one of the largest emergency evacuations in American history. Rick Scott of Florida, where Irma made landfall early Sunday as a category 4 storm with winds up to 130 miles per hour .