Stay calm, all is hell!

After every terror plot since 9/11, officials rushed to calm the public by saying the plotters were not directly working for al Qaeda or another known organization. They used phrases like “one off,” an “isolated extremist” or, in the Boston case, “self-radicalized” to downplay the threat.

Even when they were wrong — the underwear bomber had training in Yemen — the aim was to assure Americans there was no imminent danger of another large- scale horror like 9/11. The emphasis on individuals working alone was meant to suggest that even if they had succeeded, there would not have been mass casualties.

In other words, keep calm and carry on.

Boston changes everything. The first successful attack in the homeland against civilians since 9/11 seems finally to be opening eyes in Washington about a growing danger of a second wave of terrorism.

And so while President Obama’s claim that the Tsarnaev brothers were “self-radicalized” aimed to soothe, something else he said sends a very different and alarming message.

“One of the dangers that we now face are self-radicalized individuals who are already here in the United States,” the president said, adding that their plots “are in some ways more difficult to prevent.”

In other words, be afraid because we’re likely to get hit again.

His remarks were a response to whether the FBI could have stopped the bombing because Russia had warned two years ago that Tamerlan Tsarnaev was a radical Islamist. Part of Obama’s answer was self-serving — al Qaeda has been weakened — but the remainder suggests he believes it’s time to give more attention to homegrown terrorists.

Belatedly, he’s right, as the Boston case and all 16 plots disrupted in New York prove. Partly because we are on offense overseas, and because our society is open, the homeland is a relatively easy target for terrorists already here.

The idea is not new — the New York Police Department, with Mayor Bloomberg’s blessing, did a study on homegrown radicals in 2007 and its surveillance of mosques and Muslim communities was designed in response. And Long Island Republican Pete King has warned for years about the danger of radical Muslims living in the United States.

Not incidentally, Bloomberg, the NYPD and King have been accused of being anti-Islam. But Boston vindicates their foresight.

Besides, the question of whether terrorists are homegrown or have foreign links is not always so cut and dried. Repeating a pattern of other plots, the surviving brother, Dzhokhar Tsarnaev, told FBI agents that they were inspired by Internet videos of Anwar al-Awlaki, an American-born Islamist killed in Yemen in 2011. So even “self-radicalized” terrorists learn from global jihadists.

Assuming Obama’s comments mean the federal government and other cities will get more vigilant, they have a lot of catching up to do.

“Law enforcement and the military lack a well-researched and defined list of indicators and warnings associated with cases of violent extremism,” writes Cliff Watts, a former FBI agent and Army vet. In an essay for the Foreign Policy Research Institute, he argues there is often a step-by-step “process of radicalization” that begins with an introduction to an extremist ideology and ends with the resolve to carry out a violent act to serve that ideology. Watts cites “emotional triggers,” ranging from the death of a family member to professional or financial setbacks, as markers for investigators.

Creating a smarter law-enforcement checklist on radicalization is an idea whose time has come. But there is another action that must come first if the new focus is to make a difference.

As I have argued, Obama’s refusal to link Islam to the terrorists is dangerously misguided. It would bad enough if he limited his willful blindness to rhetoric.

But reports indicate that he and Attorney General Eric Holder have imposed their warped views on law enforcement through training manuals and procedures, handcuffing defenders and raising the odds that homegrown terrorists will succeed.

If Boston taught us anything, it is that one or two men can wreak havoc on a city and rattle the nation. Political correctness kills and every tool we have to stop the next Tsarnaev is a tool we must use.