President Joe Biden on Sunday marked the anniversary of the Sept. 11 attack by attending a wreath-laying ceremony at the National 9/11 Pentagon Memorial in Arlington, Virginia. Biden honored the thousands of victims who died in the worst terrorist strike in U.S. history.

"I know for all those of you who lost someone -- 21 years is both a lifetime and no time at all," Biden said in a speech. "It's good to remember. These memories help us heal. But they can also open up the hurt and take us back to that moment when the grief was so raw."

Biden commended the citizens and service members who responded to the attack at the Pentagon. "We owe you," Biden said.

Vice President Kamala Harris attended the annual memorial service in New York.

"On this solemn day, we honor those lost at Ground Zero, Shanksville, and the Pentagon twenty-one years ago today. We remember their lives and the heroism of our first responders. My heart is with those who lost loved ones," Harris posted on Twitter Sunday.

The 9/11 Pentagon Memorial was opened on Sept. 11, 2008, and was created to remember and honor the family members and friends of those who were victims of the attacks.

Biden insisted that his administration continues to make sure Americans are protected against terrorism.

"We have an obligation, a duty, a responsibility, to defend, preserve and protect our democracy. The very democracy that guarantees the rights of freedom that those terrorists on 9/11 sought to bury in the burning fire and smoke and ash," Biden said.

Nearly 3,000 people were killed and about 25,000 were injured in the Sept. 11 attacks.