Rescue workers survey the damage to the World Trade Center in New York City in this photo taken on Set. 11, 2001. The Twin Towers of the World Trade Center which were struck by hijacked airplanes collapsed on that day claiming 2,753 lives. September 11, 2016 marks the fifteenth anniversary of the event.
camera (Doug Kante/AFP/Getty Images) Rescue workers survey the damage to the World Trade Center in New York City in this photo taken on Set. 11, 2001. The Twin Towers of the World Trade Center which were struck by hijacked airplanes collapsed on that day claiming 2,753 lives. September 11, 2016 marks the fifteenth anniversary of the event.

Nicholas Casale is a retired New York City police detective, former deputy director of counterterrorism for the Metropolitan Transportation Authority and an ABC News contributor.

This week I returned to Ground Zero for the 15th Anniversary of 9/11.

Standing on the promenade I got the sense that Battery Park City had bounced back, with crowds of locals and tourists strolling along the Hudson River. Trendy restaurants were crowed with lunchtime patrons. Standing majestically in the brilliant sunlight was the Freedom Tower.

And yet I thought, can all of this be destroyed, too?

A lot has changed in 15 years and a new generation has come of age born after the attacks. Now police officers and security guards continue to patrol the area with high-powered weapons and bomb sniffing dogs. Buildings are still surrounded with concrete and steel bollards and NYPD cameras look over it all from street lamps.

The elite FBI Joint Terrorist Task Force made of special agents and NYPD detectives is hard at work following-up on the most insignificant tips. Someone somewhere is under surveillance. Smart cameras and computers are tracking vehicles and people at police headquarters. In a building near the Meat Packing District agents and detectives sit at computers searching data.

But despite all that's being done, I couldn’t shake the question: Could it happen again?

I thought back to Sept. 11, 2001. I was reading the morning paper in my apartment in downtown Manhattan, just across from the South Tower when I heard the first explosion. I looked out my window and saw a large trail of smoke and the tower engulfed in flames. I called police headquarters, but no one knew exactly what had happened yet, so I ran to my car, grabbing my police radio and windbreaker. Looking up I could see the devastation. People were leaping from the burning inferno on to the street below.

When the first tower collapsed, everything was in slow motion. It's hard to remember what happened next among the fire, smoke and rubble, but eventually some other officers and I learned ferries were helping to evacuate the injured. Another officer, Lt. Timothy McGinn, and I commandeered a tug boat and did what we could, ferrying the injured across the Hudson River to Jersey City and bringing dozens of emergency personnel back the other way. After that I went back to Manhattan to help at Ground Zero, working alongside countless others for hours to dig through the mountain of rubble searching for survivors.

New York lost more than 400 of her bravest emergency services personnel that day. The overall death toll was nearly 3,000 killed and 6,000 injured. We mourned our loses, praised our heroes and sent our soldiers, sailors, Marines and aircrews to take the fight to the enemy.

This week I thought back further as well, to another time the World Trade Center had been targeted by terrorists. It was 1993 and I was assigned to the police commissioner’s office when the call came that a bomb detonated in the parking garage below the North Tower.

West Street was filled with police cars, fire trucks and ambulances when I arrived. Police, fire and port authority bosses were barking orders and no one knew who was in charge. Power and telephone service was lost. Radio and television stations that depended on the rooftop antenna lost their broadcast single.

Then amidst the chaos then-police commissioner Raymond Kelly picked then-deputy chief Lou Anemone to take over police operations, and Anemone brought order to the scene. Police helicopters started airlifting survivors from the rooftop of the North Tower. The FDNY fought the fire and escorted thousands to safety. Later in the evening, Anemone, two emergency services cops and I descended into the garage and saw first hand the blast site. Six people were dead and 1,000 or more injured.

A small piece from a truck axle provided agents and detectives with the clue that led them to a Ryder rental store in Jersey City. In the end Ramzi Yousef and others were arrested and convicted. They had been working for al-Qaeda.

In the aftermath of both attacks on New York City, authorities issued recommendations designed to keep its people safer and the city, along with the rest of the nation, has made significant changes. But having found myself in the middle of the destruction immediately after both tragedies, I'm still worried it could happen again.

Today our security shield is based on three levels: intelligence, visible prevention and quick response. There are gaps that need to plugged: Communication can be better between police and emergency personnel in the case of a catastrophe. Power grids, freight yards, transit hubs lack smart cameras, sensors and other safety measures. Many promised upgrades have been repeatedly delayed and their costs have ballooned.

We've spent billions on security but the terrorist attacks abroad and the memory of 9/11 and 1993 have to be a recurring wake-up call. Homegrown extremists are plotting against the U.S. and we have to brace for new waves of attacks not only from abroad, but from self-inspired, disenfranchised or radicalized Americans.