It’s difficult to believe that the 9/11 terrorist attacks occurred two full decades ago. Anyone who was over the age of five that day can recount precisely where they were and what they were doing when the towers came crashing down, the Pentagon burned, and smoke rose from a smoldering crater in rural Pennsylvania.

Nearly 3,000 Americans lost their lives in the attacks—among them were 37 officers of the Port Authority Police Department and 23 officers of the New York City Police Department as well as 343 firefighters and EMTs. Many public safety professionals have died over the last 20 years because of the hazardous materials they were exposed to in the days, weeks, and months following September 11, 2001.

How has the fight against terrorism changed for American law enforcement in the 20 years leading up to 9/11. How has it changed in the 20 years since? And what can we expect for the next 20 years?

Prelude to 9/11

It’s a useful enterprise to briefly examine the history of terrorist attacks on U.S. interests in the years leading up to 9/11.

During that period, there were several notable terrorist attacks on U.S. soil by foreign entities that merit attention as they offered lessons—some quickly learned and some ignored until it was far, far too late—such as the 1990 assassination of Rabbi Meir Kahane in New York by El Sayyid Nosair and the bombing of the World Trade Center in 1993.

Chaos and confusion were abundant in the aftermath of those attacks, and the depth of doubt and the dearth of data would remain for many years.

Prior to 9/11, most successful terror attacks against American interests by foreign actors that caused large numbers of casualties and/or produced significant damage to U.S. property largely took place outside of the United States. There was the 1983 U.S. embassy bombing in Beirut, the 1998 embassy bombings in East Africa, and the suicide boat bomb attack on the USS Cole at a Yemeni harbor in 2000.

And the sad truth is that prior to Sept. 11, 2001,  most terror attacks on American soil were Americans killing their fellow Americans.

They were perpetrated by groups like the Ku Klux Klan, the Weather Underground, the Black Panthers, the Symbionese Liberation Army, the New World Liberation Front, and a host of others. Or they were carried out by lone-wolf terrorists like the Unabomber Ted Kaczynski who sent mail bombs to specific targets for 17 years, and Eric Rudolph, who was responsible for the 1996 Centennial Park bombing during the Atlanta Olympic Games and a number of attacks on abortion clinics.

By far the most deadly domestic terror attack in U.S. history happened in 1995 in Oklahoma City. The bombing of the Alfred P. Murrah federal building in 1995 killed 168 people and injured more than 680 others. It stunned 1990s America and at first many people believed it was the work of foreign agents. Then it was learned that the murders were committed by two Americans, Timothy McVeigh and Terry Nichols.

In all of these cases, bringing those responsible to justice was the work of domestic American law enforcement at the local, state, and federal level.

Then, 9/11 happened.

Countering Terrorism

It’s indisputable that 9/11 was the “tipping point” for American law enforcement efforts to stem the tide of terrorism in the United States.

In what seemed like an instant, federal, state, and local law enforcement were placed on the front lines of counterterrorism efforts on U.S. soil while American military personnel were deployed overseas to secure America against countries that harbored or gave aid to terrorist groups.

On the legislative front, Congress passed the USA Patriot Act, expanding the ability of law enforcement at every level—but especially at the federal level—to increase surveillance and searches in interdicting international money laundering aimed at financing of terrorism as well as investigating other activities connected to terrorism.

The manhunt that followed the Boston Marathon bombing showed the level of maturity police across the country had achieved in terms of tools, tactics, and training for rapid response to sudden terror attacks. Naturally there were bound to be “turf wars” between the various agencies involved in the response, but in the end, the Tsarnaev brothers were brought to justice.

After the terror attack in San Bernardino left 14 people dead and 22 others injured, the perpetrators—a married couple named Syed Rizwan Farook and Tashfeen Malik were killed in a gun battle with police. During the subsequent investigation, the FBI was pitted in another type of battle, this time with tech giant Apple over its cell phone encryption technology.

Tomorrow’s Threats

The one unchanging thing about terrorism is that it is constantly evolving—moving into uncharted waters as easily as returning to familiar and frequent ports of call. If something has never been done, someone will see that fact as a challenge. If something worked in the past, someone will see that as a clear path to success.

Airplanes as guided missiles as seen on 9/11? Sure, the tactic was refined by the Japanese kamikaze units during World War II. Using commercial aircraft for such attacks on the United States was foreshadowed in Tom Clancy’s 1994 novel “Debt of Honor” in which an antagonist character in the book steals an airliner and crashes it into a joint session of Congress held in the Capitol Building.

Fire as a weapon of mass destruction? It’s been a weapon of war for centuries. Union General William Tecumseh Sherman, who wanted the Confederates to “feel the hard hand of war,” used it very effectively in his campaigns in Georgia and the Carolinas. It’s borderline miraculous that a small cell of attackers has not yet emerged to light fires in multiple locations across the parched Western United States.

And what we’ve experienced in recent years tells us that we need to think outside the box when trying to forecast potential terror attacks.

Cyberattacks have already begun to wreak havoc on a variety of targets from the energy sector to financial institutions and other vital entities. The COVID-19 pandemic demonstrated in stark relief how a deadly virus can kill hundreds of thousands of Americans and practically bring the American economy to a standstill.

If it can be imagined, it can be done—given adequate resources and sufficient motivation.

Since the murder of more than 3,000 souls on 9/11, terrorism has been one of the most difficult challenges facing law enforcement entities worldwide.

These acts are largely conducted by actors who live and work in secret—lurking in the proverbial shadows—until they suddenly appear and attack. However, they are not unknown and they are not unknowable.

Recent events have provided ample proof—if any was needed—that the landscape of possible terrorist actions has expanded and become moredangerous.

If past is prologue—and the past is most certainly prologue—a nearly limitless array of challenges await American law enforcement.

Doug Wyllie is contributing web editor for POLICE.