Matthew Quirk, a novelist, is author of "The 500" and "Dead Man Switch."

A Colorado chlorine leak in 1971. (Bill Wunsch/The Denver Post via Getty Images)

A gray SUV idled across the street from the chemical plant. Gas storage tanks, four stories tall, towered over the low-slung neighborhood. It was a hot, dry Sunday in southeast Los Angeles.

The plant’s front gate was open. The driver tapped the throttle, then cut into the facility, past the “no trespassing” notices and the signs demanding that all trucks stop and check in with a guard. He pointed the car straight at three trailers loaded with compressed hydrogen. Behind them, on the other side of a rusting chain-link fence, rail tankers sat outside a facility that uses chlorine to manufacture bleach. One tanker car of chlorine, if ruptured (by, say, a nearby hydrogen explosion), could reach 4.9 million people in the Los Angeles Basin and kill 10,000 under worst-case conditions.

The driver veered away from the gas tanks, then stopped the car and waited. No one came to check on him as he took a few photos on his phone. After five minutes, he pulled away.

That was me. I write thrillers for a living. For my latest novel, “Dead Man Switch,” I spent a lot of time researching the materials lying around the United States that terrorists could use to kill tens of thousands of people. I like to think my books are pretty tense, but they have nothing on reality: More than 15 years after 9/11, we have failed to take basic steps to address glaring threats that have already cost American lives.

With its ongoing attempts to enact a ban on many Muslim travelers and “extreme vetting” for visitors to the United States, the Trump administration has treated terrorism as a political cudgel rather than the grave and present threat it truly represents. In the years after 9/11, there was extraordinary bipartisan momentum to identify threats and safeguard against them, but the work is unfinished. With terrorism back atop the agenda, we should spend our time and money addressing the obvious risks, not the hypothetical or concocted ones.

Chlorine is scary stuff. It hugs the ground as a slow-moving cloud of yellow-green gas, and it killed thousands as a chemical weapon in World War I. When inhaled, it turns to hydrochloric acid in the lungs, and organic material — flowers or skin — bleaches and oxidizes in its path, a slow, agonizing burn. Even without foul play, accidents are often fatal: Two trains crashed and ruptured a 90-ton chlorine tanker in Graniteville, S.C., in 2005, killing nine people and sickening 250 in an area that, thankfully, was not more densely populated.

Hijacker Mohamed Atta scouted out chemical plant attacks before 9/11, and since then, study after study has called out the risk from chlorine plants: 5 million people in the toxic-release zone in L.A., 12 million in and around New York City. But the government didn’t set new rules for chemical security until 2007. Department of Homeland Security officials identified the 4,400 highest-risk facilities, but by 2013, 78 percent of those were still waiting to have their security plans approved by DHS, and 99 percent had never been inspected.

It’s easy to beat up on DHS, but a lack of political will is really to blame. In a speech on the Senate floor during the 2006 fight over chemical security legislation, freshman Sen. Barack Obama argued: “We cannot allow chemical industry lobbyists to dictate the terms of this debate. We cannot allow our security to be hijacked by corporate interests.” Yet that is exactly what happened. Chemical companies spent tens of millions of dollars lobbying to win industry-friendly rules and watered-down regulations. They killed a proposal to encourage the use of chlorine alternatives, such as concentrated bleach, that could have dramatically reduced the need to ship poison gas around the country in 90-ton tankers. Congressional funding for the DHS chemical security program was authorized only temporarily, so it lurched from one appropriation to another.

After Congress gave the program a long-term reauthorization in 2014, it made rapid progress. According to data from a senior DHS official who spoke with me on background, 91 percent of covered facilities have had safety plans approved, and 70 percent have been inspected. But DHS still hasn’t completed efforts to make sure plant employees aren’t on the terrorism watch list — a basic safeguard launched only in 2016 and now rolled out to just 6 percent of covered facilities — or to implement similar security measures for ammonium nitrate, one of many chemical threats that are not adquately covered by the current mishmash of loophole-filled rules.

That widely available compound, commonly used as fertilizer, forms a powerful explosive when mixed with fuel. It killed 168 people in the 1995 Oklahoma City bombing, the second deadliest terrorist attacks on American soil. In 2013, a Texas fertilizer plant with a history of safety violations blew up, taking the lives of 15 people, injuring 160 and destroying 150 buildings, including a middle school. Long considered an accident, it is now under investigation as possible arson or domestic terrorism. If the explosion hadn’t occurred after work hours, many more people would have died.

That prompted the Obama administration to strengthen some EPA rules on the handling of dangerous chemicals to prevent accidents, but Republicans in the House are now moving to roll back even those minimal safety measures. Ammonium nitrate is a proven killer. DHS was charged with putting out rules to prevent it from being used in an attack 10 years ago; the agency is still studying the issue. In the meantime, someone on the terrorist watch list can still buy it in bulk.

Chemical safety isn’t the only failure. The Transportation Security Administration, our most direct answer to the 9/11 hijackings, missed 95 percent of weapons and mock explosives smuggled through in a 2015 test, and it now faces $80 million in cuts under President Trump’s proposed budget (flight security fees would increase, but the money would go to the border wall). The biodefense sensors DHS deployed after the 2001 anthrax attacks don’t work reliably, and as Steven Brill reported in the Atlantic, nearly half of the thousands of hospitals that hold radioactive materials suitable for a dirty bomb have inadequate security.

“The Bush administration put all their eggs in the ‘take the battle to the enemy’ basket, asserting that ‘the only defense is offense,’ ” says Stephen Flynn, who was the lead homeland security adviser to Barack Obama during his presidential transition in 2008, an expert adviser to the Department of Homeland Security and director of a host of blue-ribbon commissions. “This translated into homeland security being a decidedly secondary priority and their making only a token effort to advance critical infrastructure security.”

He pointed me to the 2002 “National Strategy for Homeland Security,” which says outright that, except for border security and national defense, free markets and private companies would be left to take care of homeland security without government intervention. The Obama administration took steps in the right direction, in Flynn’s view, but it wasn’t enough. “America’s critical infrastructure remains extremely vulnerable to sabotage,” he said, “and the consequences of a successful attack against assets such as chemical facilities remain potentially catastrophic.”

At the Los Angeles chlorine plant, I parked across the street and walked along the railroad tracks, between a hazmat site and a row of palms growing in a ditch. I spent a half-hour working my way around the facility, trying doorknobs, squeezing into openings in the fence and snapping photos of security weaknesses. An unarmed guard, a contractor, was posted at the main entrance, but in the back, out of view of the security cameras, only a padlock and chain secured a gate that led straight to the tankers. I leaned through and snapped a few shots of the gasworks. No one stopped or questioned me.

“That is 100 percent consistent with what I have always found, everywhere,” says Sam Faddis, a retired CIA operations officer who specialized in counterterrorism and ran similar tests on chemical, biological and nuclear targets for his book, “Willful Neglect: The Dangerous Illusion of Homeland Security.” Check-ups like mine have yielded similarly depressing results at dozens of other plants. “The security might stop a teenager from painting some graffiti,” Faddis said. “But it’s nothing that would even approach the level of stopping a terrorist who isn’t planning on living through the attack.”

Such glaring security holes at plants that could kill thousands of people are far more terrifying than any villain I could dream up. The next guy through the fence probably won’t be a writer with an overactive imagination and good intentions. Our vulnerabilities are well known, and documenting them for decades without fixing them makes us less safe than doing nothing at all.