This Monday marks 16 years since the deadliest foreign attack on American soil.

On September 11, 2001, four airliners, each bound for California, were hijacked by Islamic terrorists. The result was something worse than the most horrid of nightmares: 2,997 innocent lives gone, destruction to the Pentagon in Washington, DC and the perhaps the mightiest structures in America — the Twin Towers — toppled and in ruin in Lower Manhattan.

On the tenth anniversary of the tragedy, an unprecedented memorial was dedicated to the memory of those lost and a day later open to the public — the National September 11 Memorial and Museum at 180 Greenwich Street, New York, New York.

On Sunday, just like every year on the hallowed date, Americans gathered at the site to pay their respect, and to remember not only the innocent who were taken from us, but the hundreds of brave first responders who were killed in their heroic attempts to save their neighbors: 343 firefighters of the FDNY, 23 members of the NYPD, 27 members of the Port Authority Police Department and eight emergency medical professionals.

In case you couldn’t make it to the memorial, here’s a virtual tour below:

Here’s footage of the memorial in Shanksville, Pennsylvania, the site where United Flight 93 crashed while courageous passengers took control of the aircraft from al Qaeda terrorists.

It was headed to strike and destroy either the U.S. Capitol or the White House.

And, finally, here’s the memorial at the Pentagon. It honors the 184 people who died there.