President Trump marks 16th anniversary of Sept. 11 attacks with ceremonies at Pentagon; White House

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President Donald Trump is marking the 16th anniversary of the Sept. 11 attacks with ceremonies in and around Washington, D.C.

Trump and first lady Melania Trump led a national moment of silence on the South Lawn of the White House on Monday morning to mark the moment when the first plane struck the World Trade Center 16 years ago. The president and first lady bowed their heads as bells tolled and then placed their hands over their hearts as "Taps" played.

Later at the Pentagon, were a plane strike killed 184 people, ceremonies were also held. The ceremony included a moment of silence at 9:37 a.m., when the plane hit.

During the ceremony Trump said the nation grieves for the people "who were murdered by terrorists" 16 years ago. He issued a warning to extremists, saying "America cannot be intimidated" and those who try will join the list of enemies "who dared to test our mettle. He said when America is united, "no force on earth can break us apart."

Moments of silence have also been held to observe when the south World Trade Center tower collapsed on Sept. 11, 2001, and when United Airlines Flight 93 crashed near Shanksville, Pennsylvania.

A ceremony in Alexandria was also held in the city's Market Square in Old Town which included a tribute and a bell-ringing ceremony.

Ceremonies will be held around the country to mark the anniversary of the attacks.

The Associated Press contributed to this report.