In the aftermath of a mass shooting, hindsight kicks in quickly. Friends, acquaintances, colleagues, or teachers are often able to pinpoint moments where they realized something was wrong with the perpetrator, or simply felt uncomfortable. President Trump picked up on this idea of warning signs in a tweet Thursday morning, responding to the killing of 17 in Parkland, Florida, the day before:

With the prospects for changes to gun laws and regulations effectively frozen at the national level, the debate after shootings has often gravitated toward similar lines of discussion, often centered around mental health. Since there’s little prospect of regulating the weapons used in the shootings, the focus is on intervening before the point when someone opens fire. Yet this line of thinking is flawed, both practically and philosophically.

First, it depends heavily on retrospect. But things that seem like obvious warning signs after the fact may have just seemed weird beforehand. (People rarely really expect anyone to become a mass shooter, since statistically such attacks are vanishingly rare.) Conversely, there are thousands of people, and especially young men, who might set off warning bells—they act strangely, they’re obsessed with weapons, they engage in various anti-social behaviors—but who will never take a gun to school and open fire.

Second, even if one could more effectively sort the people who are just kind of weird from the people who might be more likely to perpetrate a shooting, what would the government do about it? Put differently, even if people “report such instances to authorities, again and again,” the authorities cannot arrest someone who has not committed a crime, simply because he makes people uncomfortable. Pre-crime is not prosecutable.

Besides, in the case of the alleged Florida shooter, the authorities were well aware of disturbing behavior. School officials had expressed concern about him, according to a teacher. He was reportedly banned from carrying a backpack at school. He was eventually expelled. (Some social science suggests that expulsion tends to create more problems than it solves, but the fact is that administrators at the school did try to do something.) He reportedly also underwent treatment at a mental-health facility. A man in Mississippi said he alerted the FBI last year to a comment made in the name of the shooter on a YouTube video, saying, “Im going to be a professional school shooter.” The FBI told the Associated Press it reviewed the comment but couldn’t find enough information to act.

In other words, even when law enforcement and other authorities are aware, it’s not necessarily enough to prevent massacres like Wednesday’s. Informing authorities is an unreliable tool, and even when it works, there are few mechanisms for authorities to act on tips they receive.

Trump said in comments Wednesday morning that in the aftermath of the shooting, he intends to focus on mental health. There’s little question that America’s mental-health system could use whatever attention, investment, and support the president intends to offer, but it’s unclear how that would forestall mass shootings.

My colleague Olga Khazan has explored this question at length. Most perpetrators of mass killings do not have diagnosable mental illnesses. Current federal law only restricts gun ownership for anyone who has been “adjudicated as a mental defective or has been committed to any mental institution”—or, roughly, those committed involuntarily to treatment in a judicial proceeding. There have been periodic pushes to widen these restrictions on the mentally ill, but most such proposals have been criticized by experts for being overly broad, encompassing large populations for which there’s no evidence of an increased propensity to commit violence, and for ignoring other risk factors—like domestic violence—that actually do correlate.  

But Trump and Sessions are unlikely to pursue broader restrictions on firearms ownership. In fact, the federal government briefly enacted a rule that people who were receiving Social Security benefits for mental illness and had been deemed unfit to handle their own financial matters could not buy a gun. But Congress passed, and Trump signed, a law overriding that one year ago.

There’s a potential paradox at play here. Gun control is politically impossible right now because a powerful group of gun advocates believes that attempts to limit ownership represent an unacceptable infringement on the right to bear arms. But since shootings like the one in Parkland, pro-gun politicians like Trump are embracing calls for surveillance and citizen informants that could represent an even greater threat to civil liberties, even as they defend the most expansive readings of the Second Amendment.

More likely, however, these calls are simply empty words. Thursday morning, Attorney General Jeff Sessions briefly touched on the shooting during a speech to sheriffs in Washington.

“It is too often the case that the perpetrators of these terrible attacks had given off signals in advance,” he said. “You are experienced professionals. You and I know that we cannot arrest everybody that somebody thinks is dangerous. But I think we can and must do better.”

Sessions did not outline what he thinks the government could do better, and that’s in part because there are few easy answers. The nation has seen a procession of deadly school shootings, and if there were a solution that didn’t involve gun control, it almost certainly would have been tried. In the absence of such ideas, Sessions is right: The government can’t just arrest everybody who seems dangerous.