Of all the echoes I still hear from the day commercial passenger jets suddenly became mass murder weapons, one resounds loudest.

It was unimaginable.

That was the first thought that occurred to me as I stood by the city desk with shocked colleagues in the Portland Press Herald newsroom and watched the World Trade Center tower belching smoke from where the first plane had just hit and then – BAM – another one hit the second tower right before our disbelieving eyes.

That we were under some kind of attack was immediately apparent. Yet even as the third jet hit the Pentagon and a fourth plummeted into the ground in southwestern Pennsylvania, an air of surrealism hung over the unfolding catastrophe.

Terrorists, armed with only box cutters, using aircraft filled with passengers to kill and destroy – and in the process, dying themselves? How could this happen? Who could imagine such a thing?

Twenty years after the attacks of Sept. 11, 2001, we’ve hardened our defenses against such an atrocity, from impenetrable cockpit doors in the air to enhanced security screening on the ground. And should someone choose to act up in flight these days, fellow passengers have no qualms about quickly subduing the troublemaker while the plane makes an emergency landing.

In short, we now not only can imagine hijacked aircraft being used as tools of destruction, we’re also hard-wired to prevent it.

Still, even after all of that, what don’t we see coming? What could happen today that, once again, turns life as we know it upside down? What new threat lurks just beyond the horizon of our imagination?

Here’s a hint: Look no further than your laptop. Or, as Sen. Angus King put it to me last week, “Why blow yourself up when you can take the grid down with a couple of keystrokes?”

Since his arrival on Capitol Hill in 2013, King repeatedly has warned of the calamity that would ensue should a bad actor – be it another nation or a band of terrorists – attack the United Sates by way of cyberspace.

“The failure to predict (the 9/11 attacks) has often been characterized as a failure of imagination,” King said. “We had little tips – the FBI knew about people taking flying lessons and not bothering to learn how to land. And there was this famous memo in August (of 2001) saying al-Qaida was determined to attack.”

Yet those red flags and others failed to upend Americans’ aura of complacency – an offshoot, in King’s opinion, of the fact that the U.S. mainland had not been attacked since the War of 1812.

“Deep in the psychology of our country was a feeling of security because of the oceans,” King said. “It was an unconscious feeling of invulnerability because we were (geographically) protected.”

Today, the threat of cyberattack knows no borders and is way beyond theoretical. Every minute of every day, hackers are hard at work looking for chinks in our digital armor, from myriad government agencies to a seemingly endless array of private-sector targets. Public water systems, gas and oil pipelines and power grids, to varying degrees, all are vulnerable. Banking systems, everyday commerce and our ability to communicate are also at risk.

Many such attacks, such as last May’s crippling of Colonial Pipeline by a since-disbanded eastern European hacking group called DarkSide, involve ransomware. Colonial paid $4.4 million to regain control of its system and end several days of panic buying at gasoline pumps throughout the Southeast.

Beyond the mercenary, however, lies even graver danger. Russia, China, Iraq and North Korea lead the list of nations looking to disrupt our elections, our economy, our national security not with bombs, but with bytes. Then there are the terrorist groups, not attached to any state, bent on destroying us outright.

From his seats on the Senate Armed Services Committee and the Select Committee on Intelligence, King sees a pattern of mutual deterrence when it comes to cyberattacks by adversarial nations: You bring down our grid, the thinking goes, and we’ll bring down yours.

“But with a terrorist organization, that calculus doesn’t apply,” King said. “My nightmare is a devastating cyberattack by an adversary against whom deterrence doesn’t work because they’re not a nation-state, they don’t have a grid to protect, there’s nothing to bomb and they don’t care about dying anyway.”

King co-chaired the Cyberspace Solarium Commission, a bipartisan group of public and private experts convened by Congress in 2019 to at least start formulating a comprehensive defense against major cyberattacks on the United States.

Patterned after former President Dwight Eisenhower’s Project Solarium – launched in 1953 to counter Cold War expansionism by the Soviet Union – the cyber commission issued a 182-page report in March 2020 that lays out a detailed framework for shoring up our cyber defenses.

Perusing it last week, two things jumped out at me.

The first is that 85 percent of our potential cyberattack targets reside outside the public sector. Meaning no national strategy can succeed unless private businesses spend the time and money needed to fortify their systems against external intrusion.

King is a vocal advocate for pen (short for “penetration”) testing, in which companies actually hire hackers to try to get past safeguards in their computer systems and thus flag their vulnerabilities.

“I talked to a friend in the energy industry,” King said, citing an example of best practices. “They send fake phishing emails to their employees every two or three months. If (workers) open it, they get a reprimand. If they do it a second time, they’re brought into the CEO’s office. And the third time, they’re out.”

Fired for opening an email? Can you imagine?

The second eye-opener comes at the very beginning of the cyberspace commission’s report. A fictional essay by 21st century warfare experts Peter Singer and August Cole titled “A Warning from Tomorrow,” it portrays the aftermath from a major cyberattack on Washington, D.C.:

The Potomac River runs red from automated systems at upstream treatment plants that were tricked into releasing the wrong chemicals.

A purplish sheen covers The Lincoln Memorial Reflecting Pool from floodwaters that covered Washington’s low-lying areas after a barrage of sensor attacks hit the area’s reservoirs.

Black smudges pockmark the National Mall where commandeered delivery drones plunged like fireballs from the sky into crowds of innocent civilians.

A tent city provides scant shelter to refugees who fled a toxic railroad accident caused by a sabotaged control system in Baltimore.

“Last night,” the essay says, “the orange of their campfires was like a vigil of the obstinate, waiting for everything to just return to the way it was.

But it won’t.”

Imagine that.