Homicidal tendencies are not evenly distributed throughout the population. Criminals, crazies and terrorists pose a heightened threat.

That’s why even those of us who strongly support the Second Amendment also support federal laws prohibiting the most dangerous among us from purchasing or possessing firearms and explosives.

There’s one problem: The restrictions in place for convicted felons and the seriously mentally ill do not apply to those on terrorist watch lists. A Government Accountability Office report found that individuals on terrorist watch lists tried to buy guns and explosives 1,228 times over a six-year period ending in 2010. In nine of out of 10 cases, the FBI could do nothing about it.

President George W. Bush recognized that this was a problem. In 2007, Bush asked Congress to give the FBI the power to block gun and explosives sales to suspected terrorists. The Justice Department endorsed the concept. Nothing came of it. Now, Rep. Peter King, R-N.Y., and Sen. Frank Lautenberg, D-N.J., have drafted legislation that attempts to close what they call the “terror gap.”

Members of the House Judiciary Committee recently voted on an abbreviated version of the bill, an amendment that would have given the Justice Department discretion to block sales to terrorism suspects on a case-by-case basis. It was voted down.

On what basis? Some members worry that restrictions on gun rights for anyone – even suspected terrorists – will erode the Second Amendment. But the King/Lautenberg bill clearly targets only those on terrorism watch lists, and it establishes a procedure so that individuals can challenge any purchase blocked by the FBI in error. In other words, if someone gets on a terrorist watch list by mistake, his right to bear arms will only be delayed, not denied.

Others who voted against the amendment raised the possibility that preventing an individual from purchasing a firearm will tip him off that he’s on a watch list.

But the King/Lautenberg bill lets the FBI decide whether it’s better to stop the purchase or let the suspect buy the gun and then see what he does.

Convicted criminals, by definition, have already committed crimes. The seriously mentally ill have been so judged by authorities. Suspected terrorists, by contrast, are just that: suspected. But they are not a suspected criminal – they are suspected unlawful enemy combatants, a very different category.

We draw up “watch lists” because we recognize that by the time we know for certain that someone is a terrorist, it is likely too late. Rare is the experienced suicide bomber.

Last week in New York, Ahmed Ferhani and Mohamed Mamdouh were arrested after they allegedly tried to buy guns, ammunition and a hand grenade from an undercover investigator.

Police said they had recorded Ferhani talking about attacking a synagogue or the Empire State Building.

If the two had gone to a licensed gun dealer they would not have been prevented from making purchases – even if their names had been placed on a terrorism watch list. The King/Lautenberg bill aims to fix that.

Also last week, a suicide bombing in Pakistan killed more than 80 people to avenge the death of Osama bin Laden.

I’ll bet the terrorists responsible would rather be in Philadelphia.

It is not difficult to imagine an attack on American soil modeled on the operation carried out by Lashkar-e-Taiba in Mumbai in 2008: coordinated shootings and bombings that left 164 people dead and more than 300 wounded.

If, one day, such an atrocity should be carried out using weapons legally and easily obtained from licensed gun stores by terrorists already on a watch list, expect a clamor for serious limitations on the right to bear arms – not just the modest precaution that King and Lautenberg propose.

“I am certainly dedicated to protecting individuals’ Second Amendment rights,” King told me. “But it defies common sense to allow suspected terrorists the ability to purchase firearms when we already prevent criminals and the mentally ill from doing so.”

In these perilous times, that’s hard to argue with – though there are those who will try.