On Thursday afternoon, Dr. Lee Silverman undoubtedly saved many lives. Using a handgun, the Delaware County psychiatrist stopped what both police and the district attorney described as a certain mass killing at Mercy Fitzgerald Hospital.

The attacker, Richard Plotts, is a convicted felon, which bans him from legally owning a gun. But Pennsylvania's universal background check law did not stop him. Neither did the hospital's signs banning guns.

The proposed federal law on expanded background checks that President Obama continually pushes is similar to Pennsylvania's and would not have stopped Plotts either. Indeed, it is hard to see how it would have stopped any of the other mass shootings during his presidency.

At Mercy Fitzgerald, caseworker Theresa Hunt was killed when Plotts opened fire during a regularly scheduled appointment with Dr. Lee Silverman. Fortunately, the doctor had his own gun and returned fire, hitting Plotts three times and critically wounding him.

After firing all the bullets in his gun, Plotts still had 39 bullets on him, bullets that he could have used to shoot many other people. Silverman's three hits, however, made it possible for two other hospital employees to tackle the wounded attacker and secure his .32-caliber revolver. Plotts has since been charged with murder and attempted murder.

Yeadon Police Chief Donald Molineux was clear: "Without a doubt, I believe the doctor saved lives. ... Without that firearm, this guy [Plotts] could have went out in the hallway and just walked down the offices until he ran out of ammunition."

Delaware County District Attorney Jack Whelan agreed: "If the doctor did not have a firearm, [and] the doctor did not utilize the firearm, he'd be dead today, and I believe that other people in that facility would also be dead."

Silverman, the hero here, apparently broke the gun-free zone rule the hospital had imposed, but he appears to be in no danger of disciplinary action and the hospital announced it was thankful for the "brave and difficult action" of Silverman and his colleagues. The hospital specifically noted: "We look forward to Dr. Silverman's return to serving patients at our hospital."

The shooting points to a sad but simple truth: The problem with gun-free zone signs, such as the one at Mercy Fitzgerald, is that the bad guys don't obey them. Instead of protecting the public, such signs only make it easier for killers to commit their attacks.

Similar endings of likely mass shootings happen regularly. For instance, a couple weeks ago in Chicago, a former soldier with a legal permit to carry stopped a gang from killing four people who had just left a party.

That action was possibly only because, slowly but surely, the folly of gun-free zones is being understood. Illinois was the last state to allow citizens to legally carry concealed handguns. If the attack in Chicago had occurred before March, the veteran wouldn't have been able to legally defend his friends.

There was further good news this weekend, when a federal judge struck down the concealed-carry ban in Washington, D.C.

Silverman's actions have drawn some criticism. DJ Jaffe, executive director of Mental Illness Policy Org, which advocates more federal spending on mental health, worried that "arming psychiatrists is more likely to be harmful than helpful" and that "violence by the mentally ill is always due to them being untreated."

But it's actually very common for potential mass killers to see psychiatrists before their attacks. The list is long, including Elliot Rodger (Santa Barbara), Ivan Lopez (the recent Fort Hood shooter), Adam Lanza (Sandy Hook Elementary), James Holmes (Aurora, Colo., movie theater) and Seung-Hui Cho (Virginia Tech). Plotts was also receiving mental-health care from Silverman.

Psychiatrists have strong incentives to get the diagnosis right. Besides professional pride and their desire to help, their own safety is at stake, as Silverman's case illustrates. In addition, they have have legal obligations to inform authorities of a threat. Families of the Aurora victims sued Holmes' psychiatrist for not recommending that he be confined.

Despite these incentives, it is still difficult to identify potential shooters, in part because the cases are so rare.

So if gun control and mental health aren't the answers, what is the ultimate last line of defense? Hopefully, Mercy Fitzgerald Hospital has learned its lesson and will take down the gun-free zone signs.

John Lott is the president of the Crime Prevention Research Center and the author of the third edition of "More Guns, Less Crime" (University of Chicago Press). johnrlott@crimeresearch.org