Trump vows to 'keep our people safe' in 9/11 speech

President Trump delivered a stern warning to extremist groups threatening the U.S. on the anniversary of the Sept. 11, 2001, terror attacks, his first commemoration of the somber occasion as president.

Speaking at the Pentagon on Monday, where one of four hijacked planes crashed 16 years ago, Trump mourned those who died and said he would honor the sacrifice by doing “whatever we must to keep our people safe.”

“The terrorists who attacked us thought they could incite fear and weaken our spirit,” he said. “But America cannot be intimidated, and those who try will soon join the list of vanquished enemies who dared to test our mettle.” 

He vowed U.S. armed forces would ensure al Qaeda and other extremist groups would never be given a “sanctuary” from where they could plot and launch a similar attack again.

“We are making plain to these savage killers that there is no dark corner beyond our reach,” the president said.

"When America is united, no force on earth can break us apart," he added.

Trump spent much of his Monday morning at remembrance ceremonies for the nearly 3,000 people killed in the attacks.

He laid a wreath at a Pentagon observance also attended by Defense Secretary James MattisJames Norman MattisOvernight Defense: Bolton heading to Geneva for Putin summit follow up | Pentagon spokeswoman under investigation | Trump statement on defense bill objects to Russia, Gitmo measures Pentagon spokeswoman investigated for misusing staff: report Hundreds gather in Yemen to mourn children killed in Saudi-led coalition airstrike MORE and Marine Corps Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff

The president, a native New Yorker, joined first lady Melania Trump in observing a moment of silence at the White House at 8:46 a.m., marking the moment when the first plane hit the World Trade Center.

Vice President Mike PenceMichael (Mike) Richard PenceLewandowski says Bloomberg would be 'very competitive' against Trump in 2020 The Hill's Morning Report — GOP seeks to hold Trump’s gains in Midwest states Former Air Force secretary: Trump's proposed Space Force is a waste of resources MORE attended a ceremony at the 9/11 memorial in Shanksville, Pa., where another hijacked plane went down.

The anniversary came as the country struggles with the widespread destruction caused by hurricanes Harvey and Irma. Trump drew a parallel between the nation’s response to the terror attacks and natural disasters, saying, “When Americans are in need, Americans pull together. We are one country.”

Trump has a complicated record when it comes to the 9/11 attacks.

As a candidate and private citizen, he has praised police officers and firefighters in New York who risked their lives to save people trapped inside the World Trade Center. He told the story Monday of a man who showed similar bravery at the Pentagon on the day of the attack.

But he has also made several divisive comments about the events of that day.  

During a debate last February, he appeared to blame President Bill ClintonWilliam (Bill) Jefferson ClintonGiuliani: Trump legal team ready to argue Mueller subpoena 'before the Supreme Court' In trade war, US has economic edge, but China has political advantage Will White House press briefings return to what they used to be? MORE for the attack because he “didn’t kill Osama bin Laden when he had the chance to kill him” and said his successor, George W. Bush, failed to do so because “he didn’t listen to the advice of the CIA.”

Trump also claimed that when the Twin Towers collapsed, “thousands and thousands of people were cheering” in New Jersey, where he said there are “large Arab populations.” There is no record of such celebrations in New Jersey. The website PolitiFact rated the claim “Pants on Fire,” denoting the most egregious falsehood. 

During a phone interview with a New York television station on the day of the attack, Trump claimed the destruction of the World Trade Center made a building he owned “the tallest” in lower Manhattan.