A Washington Post fact check agreed with Sen. Marco Rubio's claim that reforms to gun laws would not have prevented any of the recent mass shootings in the U.S. But both Rubio and the Post are wrong: the assault weapons ban proposed after the shooting at Sandy Hook Elementary School would have limited the availability of the guns used in at least two recent high-profile mass shootings. And the Post downplayed or ignored other relevant gun violence prevention legislation that could have prevented -- or at least mitigated -- other recent tragedies.

During a December 4 interview on CBS, Rubio asserted that “None of the major shootings that have occurred in this country over the last few months or years that have outraged us, would gun laws have prevented them.” Washington Post fact checker Glenn Kessler highlighted Rubio's claim in a December 10 piece, noting that a colleague had flagged the comment for a potential fact check under the assumption that “it was almost certainly incorrect.” Kessler, however, awarded Rubio's claim “a rare Gepetto Checkmark,” arguing that it “stands up to scrutiny.”

In several instances of mass shootings cited as evidence for why gun laws wouldn't work, Kessler gave an incomplete account of the role gun laws could have played in preventing or reducing the amount of deaths.

For example, Kessler claimed that the sale of a handgun to Dylann Roof, who shot and killed nine people at a church in Charleston, South Carolina, was an “example of an existing law that apparently failed.” Roof was allowed to purchase a gun because a clerical error resulted in the examiner not seeing a charge resulting from Roof possessing the drug Suboxone without a prescription, even though the seller knew an arrest record existed.

But Roof was able to purchase the gun due to a loophole in the gun law. A “default proceed” sale allows the purchase of a firearm at the discretion of the merchant, thanks to an NRA-backed amendment added to the 1993 Brady bill that created the background check system. In October, two Democratic senators introduced a bill to close the default proceed loophole.

While Kessler originally asserted that “some analysts believe Roof actually would have passed the background check if it had been done correctly” and that the FBI had “incorrectly referred to a felony drug charge” in its statement, he updated to note that the FBI still says Roof “would have been denied a gun based on an 'inference of current use.'” (This update alone arguably debunks the entire premise of giving Rubio a “Gepetto checkmark.”)

Kessler acknowledged that the proposed assault weapons ban of 2013 would have stamped out the availability of the Bushmaster XM-15 rifle that was used by Adam Lanza in the Newtown massacre, but wrote that “gun-control proposals would not have prevented Lanza's theft of his mother's legally obtained firearms.”

While the ban would not have affected the Savage Mark II rifle that Lanza used to kill his mother and the handguns and shotguns that were found in his car, the main weapon he used to kill 26 people would not have been available. The high capacity magazines that Lanza used - capable of holding 30 rounds - would also not have been available thanks to a law passed by Connecticut after the tragedy.

Kessler also wrote that California's gun laws “did not thwart” Syed Farook and Tashfeen Malik when they shot and killed 14 in San Bernardino, California.

But again, a federal assault weapons ban would have cut off -- or at the very least, limited -- the availability of the AR-15 rifles and high capacity magazines used in the crime.

While he claimed “the Fact Checker obviously takes no position on proposed gun-control laws,” Kessler nonetheless cited discredited gun advocate John Lott Jr. to bolster his conclusion that an assault weapons ban would not have had an impact the gun crimes in question. Lott, author of the book More Guns Less Crime, has often been the go-to source for downplaying the problems caused by gun violence. He has repeatedly produced unscientific research and made factual distortions in order to make his case, and has even been accused of completely fabricating evidence.

Kessler also cited Gary Kleck, who has repeatedly been cited in the right-wing media based on his research that increased availability of firearms makes people safer. But Kleck's most-cited study -- which asserts that guns are used defensively roughly 2.5 million times per year -- has been criticized for having “serious methodological difficulties.” In a 1997 paper, Harvard Injury Control Research Center Director David Hemenway explained how Kleck's data is implausible:

[I]n 34% of the times a gun was used for self-defense, the offender was allegedly committing a burglary. In other words, guns were reportedly used by defenders for self-defense in approximately 845,000 burglaries. From sophisticated victimization surveys, however, we know that there were fewer than 6 million burglaries in the year of the survey and in only 22% of those cases was someone certainly at home (1.3 million burglaries). Since only 42% of U.S. households own firearms, and since victims in two thirds of the occupied dwellings were asleep, the 2.5 million figure requires us to believe that burglary victims use their guns in self-defense more than 100% of the time. [emphasis added]

In several of the other mass shootings cited by Kessler, he acknowledged the use of assault weapons in the commission of the crime but dismissed the idea that a proposed federal ban would have prevented the mass killings, or at least reduced the amount of deaths.

Kessler also claimed that the evidence that a ban on high capacity magazines would reduce the amount of dead in shootings is “heavily disputed,” but studies have shown that weapons utilizing high capacity magazines are involved in a disproportionate amount of mass shooting incidents. In the Sandy Hook shooting, for example, several children were reportedly able to escape while Lanza paused to reload. Nicole Hockley, whose son was killed at the school, has argued, “We ask ourselves every day -- every minute -- if those magazines had held 10 rounds, forcing the shooter to reload at least six more times, would our children be alive today?” (The 2011 mass shooting in Tucson, which falls outside the time period assessed by Kessler, was stopped when shooter Jared Loughner paused to reload his gun.)

Kessler nods to, but dismisses, the idea that banning high-capacity magazines might reduce gun deaths in mass shootings, claiming, “It is possible that some gun-control proposals, such as a ban on large-capacity magazines, would reduce the number of dead in a future shooting, though the evidence for that is heavily disputed.”

In his fact check, Kessler fails to mention the Isla Vista attack, where Elliot Rodger killed six people before killing himself. Following Rodger's rampage, California passed legislation “allowing the temporary seizure of guns from people determined by the courts to be a threat to themselves or others.” The legislation could have stopped Rodger from carrying out his plan - as the LA Times notes, the massacre took place “even though the family of Elliot Rodger had sought help because of concerns about his strange behavior before the shootings.”

Similar legislation has been introduced in Congress. Laws like this could feasibly help prevent shootings where the gunman's family or associates knew they were disturbed but were powerless to prevent them from accessing guns.

Again and again throughout his piece, Kessler chose to play up the gun advocate position in each case without acknowledging the practical effect of gun laws and remedies that have been opposed by groups like the NRA and elected officials like Senator Rubio.