Four years ago Wednesday, 20-year-old Adam Lanza took a Bushmaster XM-15 to Sandy Hook Elementary School and unloaded 154 bullets in five minutes, killing six adults and 20 elementary school kids. At the time, the 26 victims made it the second worst shooting in American history.

Two days later, Sen. Dianne Feinstein (D-Calif.) appeared on “Meet the Press” to announce her intention to introduce a bill banning assault weapons ― a prohibition that was part of federal law from 1994 to 2004, but that Congress let expire. The Sandy Hook massacre was the third time in a little more than a year that someone had shot up a public place using a type of weapon that was previously banned.

“We can’t ignore the common denominator in all of these deadly massacres: access to easy killing machines,” Feinstein told reporters several days later. “Sandy Hook is only the latest tragedy and more horrendous than anything I ever thought could happen in the United States of America. But these massacres are happening … and the only thing that’s consistent in all of them are the guns.”

We can’t ignore the common denominator in all of these deadly massacres: access to easy killing machines. Sen. Dianne Feinstein (D-Calif.)

The Dec. 14, 2012, rampage in Newtown, Connecticut, was also the 28th mass shooting ― which the FBI has defined as the public killing of four or more people in a single incident ― since the assault weapons ban had expired eight years earlier. But unlike previous acts of indiscriminate mass murder that had failed to stir Congress, the Sandy Hook tragedy felt like it could inspire action.

Which is to say, it felt like that moment 19 years earlier when Feinstein had first introduced an assault weapons ban, four months after a man armed with two TEC-9 semi-automatic pistols rode up the elevator of a skyscraper in San Francisco’s Financial District, got out on the 34th floor and opened fire, killing eight people and wounding six. The 101 California Street incident was the fourth mass shooting in California in less than a decade.

In a speech on the Senate floor soon after, Feinstein argued that no civilian should have access to the kinds of guns used to mow down her constituents.

“They are weapons of choice,” she said, for killers who want to take out their wrath “on anyone who happens to be around — children in a school yard or a swimming pool or walking down a street.”

The 1994 assault weapons ban became only the third piece of gun control legislation approved since 1968, but massive resistance left the law riddled with so many exemptions it was basically useless. The director of federal affairs at the National Rifle Association, the country’s top lobbying group for gun owners, portrayed the bill as an effort to “disarm” the American public, arguing in congressional testimony that advocates of the ban were “merely interested in eliminating any type of firearm whenever presented with an emotionally charged opportunity to do so.”

In an effort to appease the gun lobby and assure passage of the bill, Feinstein whittled down the list of firearms to be banned from 670 to 19. The TEC-9s used in the 1993 San Francisco shooting made the list, but the law exempted any weapon manufactured before the ban took effect from any restrictions.

The company that made TEC-9s, Navegar, ramped up production as the bill moved through Congress, and in the first eight months of 1994, it produced three times as many guns as it had in 1992. The company was allowed to sell every single one of those, and anyone who had purchased one could legally resell it to whomever they wanted.

That’s how Feinstein’s seemingly momentous assault weapons ban set the script for every other ineffectual attempt to implement gun laws in the United States. It unfolds the same way each time: A horrifying shooting happens and lawmakers pledge to prevent the next one. But often nothing comes out of Congress. On the rare occasions legislators do act, resistance from the gun lobby leaves the new law downright impotent. The massacre is soon relegated to a place on a list we recite after the next mass shooting, which starts the process anew. For more than two decades, it’s been America’s consistent cycle.

That’s how a gun store in Illinois could sell a TEC-DC9 ― a weapon ostensibly illegal in the U.S. ― three years after the assault weapons ban took effect. And it’s how, two years after that, the same weapon was legally available at a gun show outside Denver to two students from Columbine High School.

MARK LEFFINGWELL/AFP/Getty Images People and press gather at a memorial outside Columbine High School, the scene of a mass shooting on April 20, 1999.

Even though they were only 17 in January 1999, Dylan Klebold and Eric Harris didn’t face any real legal impediments to obtaining guns.

The Brady Handgun Violence Prevention Act, a law first floated after President Ronald Reagan was shot in 1981, didn’t pass until 1993. It granted the Justice Department five days to perform a background check before a handgun sale could go through. A law barring felons, juveniles and the mentally ill from buying weapons had been on the books since 1968, and the Brady bill was designed merely to give law enforcement a way to enforce it. Still, the NRA repeatedly attempted to sink the bill, arguing that it “violated Americans’ constitutional right” to have immediate access to firearms.

To appease the gun lobby, Congress agreed to create the National Instant Criminal Background Check System. The NICS, which went into effect in November 1998, required licensed gun sellers to check a prospective firearm buyer against a government database. The promise was that they would receive a response within two minutes on 90 percent of those inquiries. The NICS was actually able to give an immediate response about 70 percent of the time in its first year of operation. On any sale that did not receive an immediate response, federal authorities still had three business days to complete the background check.

But the Brady law explicitly excluded transactions between private citizens, including transactions between people at gun shows. The so-called gun show loophole means that Americans can buy guns with no background check whatsoever.

That’s how the two Colorado teenagers purchased four weapons over the course of two months. On April 20, 1999, they brought those guns to Columbine High School and, in less than an hour, fired 181 shots, including 96 from the TEC-DC9. Klebold and Harris killed 13 and wounded 21, the fourth-highest victim total in a mass shooting at the time.

Then-Rep. Rod Blagojevich (D-Ill.) had introduced a bill addressing the gun show loophole just 16 days before the duo purchased their most lethal firearm. Gun show participants, he said in a floor speech, had “little, if any, legal obligation to insure that they aren’t putting deadly weapons into the hands of violent criminals or juveniles.” He had introduced the same bill the previous year, with no success.

The Columbine shooting might have given impetus to Blagojevich’s legislation. Instead, Sen. Orrin Hatch (R-Utah), who chaired the Senate Judiciary Committee, saw it as an opportunity to revive his own stalled bill aimed at harshly punishing teens repeatedly convicted of crimes. In asking that his measure be brought to a vote 21 days after the shooting, Hatch shifted the blame away from guns and onto the teens themselves.

“In the face of a confounding problem like juvenile crime and school violence, it is tempting to look for easy answers,” Hatch said on the Senate floor. “It is at its core a problem of our nation’s values. ... We should not simply rush to enact more gun control ― some of which cannot even be remotely associated with the [Columbine] tragedy.”

But the main gun control push was for a measure that actually focused on what had gone wrong at Columbine. The late Sen. Frank Lautenberg (D-N.J.) proposed an amendment to Hatch’s legislation that was similar to Blagojevich’s bill. It would have required all firearm purchases at gun shows ― even those between private sellers ― to be run through the background check system and would have granted the government the same three days to conduct those checks for sales on which they could not give an immediate response.

We should not simply rush to enact more gun control. Sen. Orrin Hatch (R-Utah)

Hatch and other Republicans, expecting a fight, had already attached an amendment that would limit the time for gun show background checks to 24 hours. The Republicans were livid that Lautenberg and his fellow Democrats wanted to give authorities the same amount of time to conduct gun show background checks that was already prescribed for every other gun purchase. Hatch said their proposal would be tantamount to “doing away” with gun shows.

The senators split 50-50 on Lautenberg’s amendment, leaving then-Vice President Al Gore to cast the deciding vote in favor. “We’re reminded again that until we get more controls in a sensible way on the easy availability of guns in our society to children, to criminals, to those who are mentally disturbed, then these tragedies will continue,” Gore said after the vote.

A month later, the Republican-controlled House of Representatives passed a version of Hatch’s bill that did not include any of the gun show language.

The gun lobby cheered. “Nothing is better than anything,” said James J. Baker, the NRA’s chief lobbyist at the time.

When the two chambers met that August to work out the differences between their juvenile justice bills ― a process known as reconciliation ― Hatch tried to keep the Senate version, the one with the gun show language, from becoming law. After one sitdown, House and Senate negotiators never met again.

The bills languished for nearly a year, until the first anniversary of Columbine. Another shooting death, this time of a Michigan first-grader at the hands of a fellow student, prompted President Bill Clinton to admonish Congress. “I don’t want any future president to have to go to Columbine or to Springfield, Oregon, or to Jonesboro, Arkansas, or to any of the other places I have been,” he said at a gun safety rally in Denver.

But the only legislation crafted in response to Columbine never went anywhere.

Kevin Lamarque/Reuters President George W. Bush speaks at Virginia Tech on April 17, 2007, a day after the mass shooting there.

Seven years later, President George W. Bush boarded Air Force One to fly to Blacksburg, Virginia, the day after Seung-Hui Cho, a 23-year-old Virginia Tech student, gunned down 32 teachers and students on campus. The April 16, 2007, mass shooting incident was then the worst in U.S. history.

Cho had once been involuntarily institutionalized, and a judge had described him as a danger to himself. But because reporting such mental health records to the NICS was voluntary, Cho was able to pass two separate background checks in the winter of 2007 and purchase two handguns.

Soon after the Virginia Tech massacre, Congress began debating whether anything could have been done to prevent such horrors. But lawmakers already knew the answer. Five identical bills introduced in the early 2000s could have prevented Cho from buying those guns. The legislation would have provided grant money to states to help them report their backlog of mental health records to the NICS. Many states were either unable or unwilling to comply with reporting requirements, which severely undermined the system’s usefulness. Indeed, in the NICS’s first year of operation, all 50 states combined sent in disqualifying mental health records for just 41 people.

One bill providing those grants passed the House in 2002, but the Senate refused to take it up. Four identical efforts never made it to a vote. It was only after the massacre in Blacksburg that Congress passed the NICS Improvement Act of 2007, authorizing nearly a billion dollars in state grants.

Yet most of that money never went anywhere. To make the bill more palatable to the NRA, congressional Democrats had added a provision, absent in the bill’s previous iterations, that forced states that took the funding to create a process allowing mentally ill people to regain their gun rights. Many states balked. Just 7 percent of the pledged $875 million was distributed over the next five years.

The NICS Improvement Act also made it harder to disqualify mentally ill people from buying guns. Previous law had loosely stated that gun sellers could not sell a firearm to someone they had “reasonable cause” to believe had been “adjudicated as a mental defective” or “committed to a mental institution.” But the 2007 law explicitly barred the federal government from simply entering a diagnosis from a licensed mental health professional into the NICS; that diagnosis had to be backed up with a court ruling that someone posed a severe threat.

In its final form, there was “far more bad in this bill than good,” Kristen Rand, legislative director of the Violence Policy Center, told the Los Angeles Times. In a press release, she called it “now nothing more than a gun lobby wish list.”

“The end product is a win,” said the NRA in its own press release.

With the schedule we have, we’re not going to get into the debate on gun control. Sen. Harry Reid (D-Nev.)

The heightened standard for being deemed too mentally unstable to have a gun is one reason why 24-year-old James Holmes ― who had repeatedly told his psychiatrist that he thought about killing people, that he thought about killing people multiple times a day, and that his desire to kill people was growing ― faced no federal legal barrier preventing him from buying a shotgun, a semi-automatic rifle, two handguns and 6,300 bullets.

Holmes opened fire in an Aurora, Colorado, theater on July 20, 2012, killing 12 and wounding 58 people. The dead and the injured together constituted the highest victim total in a mass shooting incident in U.S. history at the time.

After federal authorities revealed that Holmes’ bulk ammunition purchases had all been made through the internet, the legislative response to the tragedy targeted online ammunition purchases. Ten days after the shooting, Sen. Lautenberg introduced a bill requiring a photo ID to purchase bullets, which would have effectively ended online sales. But it was the middle of summer, and Sen. Harry Reid (D-Nev.), then the majority leader, said there wasn’t enough time to take up the measure, or any of the others that were floated in response. “With the schedule we have, we’re not going to get into the debate on gun control,” Reid said.

Three days later, the Senate took its annual month-long recess.

It probably didn’t help that a presidential election was only months away. Even Feinstein spoke with restraint. She noted that one of the weapons Holmes used was a Smith & Wesson semi-automatic rifle introduced in 2006, after her assault weapons ban had expired, and said she supported the ban’s reintroduction. But she said, “I don’t believe it has a chance in this environment.”

PAUL J. RICHARDS/AFP/Getty Images Sen. Dianne Feinstein tried to bring back the assault weapons ban after the Sandy Hook shooting.

Feinstein’s new assault weapons legislation, proposed a month after the December 2012 Newtown shooting, focused on 157 specific types of semi-automatic rifles. The bill explicitly exempted 2,200 gun models, to give it any hope of passing.

Her announcement on “Meet the Press” prompted NRA Executive Vice President Wayne LaPierre’s oft-quoted statement that “the only thing that stops a bad guy with a gun is a good guy with a gun.”

Alongside Feinstein’s ban, Sen. Charles Schumer (D-N.Y.) introduced a bill that would have expanded background checks on most private sales of guns in the United States. It was essentially identical to a proposal President Barack Obama had just made. That measure incensed the NRA, which claimed the bill would ban loaning firearms to friends and gifting one’s kids with guns, despite the generous exemptions carved out for precisely those situations.

“[Obama] wants to force parents to fill out forms to leave a family heirloom to a loved one — standing in line and filling out a bunch of bureaucratic paperwork, just so a grandfather can give a grandson a Christmas gift,” LaPierre said.

Even in the immediate wake of a mass killing at an elementary school, getting a bill to the floor was a fight. The assault weapons ban passed a vote in the Senate Judiciary Committee, but when Democrats reached a compromise to bring Schumer’s background check measure to the floor, Reid left the ban behind. He assured Feinstein there would be a vote on the assault weapons ban as an amendment. Lautenberg was also going to introduce an amendment barring the type of high-capacity magazines used in both Aurora and Newtown.

A mere title in the Democrats’ plan incensed Republicans: “Requiring a background check for every firearm sale.” But Democratic leadership allowed the bill to move forward with the understanding that a less aggressive, bipartisan compromise, which Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.) were working on, would be introduced as an amendment replacing the bill’s central language. The Manchin-Toomey effort was meant to placate the vocal minority against expanding background checks, by focusing solely on internet purchases and gun show sales. The pair also gave their version a revised title: the Second Amendment Rights Protection Act.

It was clear that any gun bill was going to face a tough vote. “In my opinion, adopting mandatory federal government background checks for purely private transactions between law-abiding citizens puts us inexorably on the path to a push for a federal registry,” said Sen. Ted Cruz (R-Texas).

And even if there were background checks, Sen. Mike Johanns (R-Neb.) said on the floor, they “would not have prevented any of the recent atrocities that have affected families in our nation.”

Sen. Chuck Grassley (R-Iowa) argued against the proposed assault weapons ban by pointing to the impotence of the previous law, despite changes that would have made the new ban more robust. “It did not stop Columbine,” he said.

The Manchin-Toomey measure and Feinstein’s amendment both came to the floor on April 17, 2013, one day after the anniversary of the Virginia Tech shooting and three days before the anniversary of the Columbine massacre.

Manchin-Toomey was the first up, and it received 54 votes ― six short of the 60 needed to overcome a Senate filibuster. Of all the proposed measures, it had the best chance of passing, and its defeat set the others up for certain failure. Lautenberg’s high-capacity magazine ban got 46 votes. Feinstein’s amendment received only 40.

A little less than two years after those votes, SIG Sauer released its new MCX semi-automatic rifle, which would have been illegal to manufacture under Feinstein’s ban. On June 4, 2016, 29-year-old Omar Mateen purchased one at a gun shop in Port St. Lucie, Florida. Seven days later he walked into the Pulse nightclub in Orlando and shot 102 people. Fifty of them died.

It was the worst mass shooting in U.S. history.

CORRECTION: An earlier version of this article said that the Manchin-Toomey measure fell four votes short of overcoming a Senate filibuster. It was six votes short.