EIGHTEEN YEARS later, the horrific memories of September 11, 2001 remain indelibly ingrained.

Who could forget where we were when we first saw the grainy images of the planes piercing the cloudless New York City sky? Or the surreal hours that followed as we grappled to comprehend the scope of the attack and all those we lost at the World Trade Center, the Pentagon, and in Pennsylvania?

No matter how many years pass, we must come together on this day to remember the victims, including a number of our fellow Granite Staters; to honor the first responders who risked everything to save others; and to thank the members of our armed forces who have fought, and continue to fight, to protect their fellow Americans in the years since.

But it’s not enough to simply remember. We must also work to prevent the horror of that day from ever happening again.

On Monday, the Senate Homeland Security Committee, on which I serve, will convene a field hearing in New York City to hear from three former secretaries of the Department of Homeland Security — who served under Presidents of both parties — about the state of homeland security today.

Our country continues to face new and evolving threats. Despite defeats on the battlefield, terrorist groups like Al-Qaeda and ISIS persist in their efforts to radicalize and recruit people from all over the world — including inside the U.S. — to carry out attacks against civilians.

And domestic terrorism of all types is becoming more frequent and more deadly.

I’m focused on putting differences aside to address rising security challenges, and am working across the aisle in a number of ways to ensure that we can stay one step ahead of those who seek to do us harm.

We must prevent terrorists — including foreign fighters of ISIS and Al-Qaeda — from reaching American soil in the first place.

One way we can do that is by strengthening a key line of defense known as the Department of Homeland Security’s Visa Security teams. These teams consist of trained counter-terrorism professionals currently stationed at some of our embassies who aid the State Department in decision-making about granting U.S. visas to foreign individuals.

We must ensure that these Visa Security teams are at every U.S. embassy and consulate that issues visas. We also need to make sure that these teams are well funded and fully equipped with the latest technology to identify possible terrorist travel, and I will keep working to make that a reality.

Unfortunately, even if we can physically keep foreign terrorists out of our homeland, attacks by homegrown terrorists of all stripes remain a real threat.

I have consistently pressed Department of Homeland Security officials on their strategy and implementation of policies when it comes to addressing domestic terrorism, and recently joined with Republican Sen. Chuck Grassley of Iowa to get answers on what the department is doing to prevent and mitigate this threat to public safety.

I am also exploring ways that we can improve how information is shared between and among federal agencies and state and local law enforcement in order to combat future attacks.

In addition, I remain concerned about terrorist offenders — such as convicted American Taliban fighter John Walker Lindh — who have been released or are set to be released from U.S. federal prisons over the next several years.

Some of these individuals have continued to openly call for extremist violence and we must ensure that the government is taking sufficient steps to keep the public safe. I joined with Republican Sen. Richard Shelby of Alabama to request information about the threat that these individuals may pose, and I am exploring what additional steps we should take to reduce terrorist recidivism and ensure that our communities are safe from these individuals.

We will never forget the horror our country experienced on September 11. And as we look toward the future, we will continue to face new and challenging threats.

Remembering is important, but it is not enough. We must summon the common purpose that has defined us in our most trying times, and work together to keep America safe, secure, and free.

Sen. Maggie Hassan represents New Hampshire in the U.S. Senate. She is a former governor of the Granite State.