Last week Sayfullo Saipov, who was approved as an immigrant in 2010, used a pickup truck to murder eight people on a bike path in Manhattan. This week Devin Kelley, who was repeatedly approved as a gun buyer in recent years, used a rifle to murder 26 people at a church in a small Texas town.

The deadliest terrorist attack in New York City since 9/11 and the deadliest mass shooting in Texas history illustrate the limits of screening as a defense against violence. We would like to think that the right combination of exclusion criteria and background checks can reliably prevent mass murder, but experience tells us otherwise.

Responding to Saipov's attack, Donald Trump promised on Twitter that "the United States will be immediately implementing much tougher Extreme Vetting Procedures," because "the safety of our citizens comes first!" But it is hard to imagine what procedure could have predicted Saipov's seven-year journey from eager immigrant to Islamic terrorist.

According to the Uzbek government, Saipov was raised in Tashkent by an affluent family and never had any trouble with the law or gave any indication of extremism. As the winner of a diversity lottery visa, he underwent background checks, security screening, and interviews before entering the United States.

Saipov, who had worked as an accountant for a hotel in Tashkent, hoped to get a job in the hospitality industry despite his limited English skills. Instead he ended up working as a truck driver, moved around a lot, and became increasingly embittered and alienated over the years.

Although Saipov was not very observant at first and did not know much about his religion (according to a local imam), he was drawn to Islamic extremism. The path he followed was shaped by his post-immigration experience, and he might never have been radicalized if he had landed the sort of job he wanted or if the trucking businesses he started had been more successful.

Kelley, by contrast, showed clear signs of violent tendencies years before he opened fire on parishioners at the First Baptist Church in Sutherland Springs. As an airman in 2012, he was convicted of beating his wife and son by a general court martial, which punished him with 12 months of confinement, a reduction in rank, and a bad conduct discharge.

Under federal law, Kelley was triply disqualified from buying a gun: His assault on his wife was the equivalent of a misdemeanor involving domestic violence, his aggravated assault on his son was the equivalent of a felony, and his separation from the Air Force, since it was ordered by a general court martial, was the equivalent of a dishonorable discharge. But the Air Force did not report Kelley's convictions to the National Crime Information Center, so they did not show up in the FBI's background checks when he bought his weapons.

The Air Force is investigating what went wrong in this case and promises to improve its reporting, which until now seems to have been limited almost entirely to dishonorable discharges. But even an improved database cannot be expected to have much of an impact on mass shootings, since the perpetrators of such crimes typically do not have disqualifying criminal or psychiatric records.

The idea that screening can prevent mass shootings is nevertheless powerfully appealing. After the October 1 shooting that left 58 dead in Las Vegas, Sen. Christopher Murphy (D-Conn.), who argues that Congress is complicit in gun violence because of its failure to "do something," told CNN's Jake Tapper "the most important intervention is background checks."

Murphy wants to require background checks for all gun transfers, not just those involving federally licensed dealers. But as Tapper pointed out, the Las Vegas shooter "passed his background checks" because "there didn't seem to be any reason to prevent him from purchasing firearms."

Even when screening demonstrably fails to stop mass murder, it does not lose its appeal among those who crave simple solutions.

© Copyright 2017 by Creators Syndicate Inc.