​San Bernardino shooting: Another all-too-familiar tragedy

It's of course not the first time a family has conspired against innocents. It was two brothers, after all, who planned the terrorist bombings at the Boston Marathon more than two years ago.

But it's the frequency of mass casualty attacks, inspired by terrorism or not, that has left us all on edge.

It's a dreadful calendar of violence. In fact, by one count, there have been 353 mass shootings in the U.S., with four or more victims, this year. On average that's more than one a day.

"We should never think that this is something that just happens in the ordinary course of events," said President Obama, "'cause it doesn't happen with the same frequency in other countries."

Historian Walter Isaacson worries not just about more attacks, but how it stains the welcome mat this country has so often laid out.

"I think a lot of things are being endangered by this event, and one of them is our basic and fundamental creed that we're an inclusive nation," he said.

Racial and religious prejudice is nothing new in America. But on the positive side, Isaacson says, we've usually found our footing.

"I think we're in a very dangerous time," said Isaacson, "but I think we've gone through everything from civil rights movements to civil wars, where there was a lot of unrest; where there were church bombings; where there were shootings and lynchings. I think that we have to not overreact, and we have to say, 'If we stick to the fundamental values embedded in our Constitution and the DNA of our nation, we're not going to let these things unravel who we are.'"

Perhaps we're not unraveled, but the country is frayed, and divided over just what to do.

Guns are a constant. In fact, holiday shoppers bought a record number of them this past Black Friday.

The procession of grief is almost guaranteed to continue. More funerals, more candlelight vigils.

Perhaps most unsettling? There's an alarming sameness to it all, when these dreadful events should be the horrific exception.