Once again the nation is grieving over a mass shooting that has taken 14 lives, leaving loved ones to wonder what contemptible mind conceives a plan to slaughter county health workers gathered for a holiday party.

The roll call of mass shootings lately has been numbing, but there are elements that set the one in San Bernardino, Calif., apart. First, the obvious: Early reports on the perpetrators embody the deepest fears of a nation engaged in a yearslong conflict with terrorists. Syed Rizwan Farook did not slip into this country undetected. He was born here, raised in Southern California and educated in American schools. And yet, early indications show he became radicalized. FBI officials already have found evidence that he had contact with Islamist extremists in the U.S. and while traveling in the Middle East.

There is no evidence that the .223-caliber assault-style rifles and semiautomatic handguns Farook used were stolen or bought on the sly. Two of the weapons, in fact, have been traced by federal officials and found to have been purchased legally. Undetonated pipe bombs were found at the site, and at Farook’s home, a makeshift weapons lab with a dozen more pipe bombs and 3,000 rounds of ammunition.

Farook was no disaffected drifter: He had a steady, well-paying job with the county that he’d held for five years and a six-month-old baby with Tashfeen Malik, who left Pakistan and entered the U.S. on a “fiancée visa” to become his bride and who helped him carry out a targeted, military-style assault on co-workers.

As the U.S. deals with a growing Muslim population in its midst, it bears repeating that as a group they are overwhelmingly peaceful and law-abiding and that many have developed a passionate love for their adopted country. In the days to come, Americans must do their best to shut out the Islamophobic invective and hatred of those who would use this trauma for their own objectives.

However, they also must be courageous enough to deal with the new reality of the widening conflict with Islamic extremists. Efforts to reach out to disaffected youth and to build community ties are vital and should continue. But such efforts are not a foolproof safeguard against the desire some harbor to destroy this country from within. As details continue to unfold, the nation should be cautious, reasoned, but unflinching in its determination to protect law-abiding citizens — of all creeds.