We've crossed a new threshold in America. While residents of Boston and environs sheltered in place last week -- hunkering down at home while police hunted for terrorists -- Americans everywhere confronted the unsettling possibility that safety within our own borders is perhaps more illusory than we had imagined.
The specter of homegrown Islamic jihadists -- young men who attended our schools, played our games, lived in our neighborhoods and assimilated at least some of our ways -- has pushed public concern to new levels.
The idea of terrorists who are integrated into our society isn't new. There was Faisal Shahzad, of Bridgeport, Conn., for example, who was sentenced to life for his attempt in 2010 to detonate a bomb in Times Square.
Shahzad's hapless failure may have made his actions easy to dismiss. That's a mistake we can't afford to repeat.
Had witnesses not seen Shahzad's smoldering car and summoned help, had his curbside bomb actually gone off, the result in Times Square would have been nightmarish.
Terrorist acts by residents accepted as American raise the stakes immensely for law enforcement. Agencies can't lock down our streets indefinitely or watch every inch of the region's sprawling transportation system. They can't search every car using using the Queens Midtown Tunnel or the Verrazano-Narrows Bridge.
At the same time, they can't let a terrorist slip by.
The case of the Tsarnaev brothers also renews our post-9/11 vulnerabilities. We must be prudent without panicking. We must be careful without bringing life as we know it to a halt. If we see something, we do need to say something. But we can't shut down the entire Long Island Rail Road main line every time someone abandons a paper bag on a platform.
We must be careful without getting crazy. And police must stay alert to danger without unduly intruding on our privacy. In the wake of the Boston bombings, Gov. Andrew M. Cuomo said that what once seemed to be random acts of violence may now reflect "the new normal." The future has arrived, and it demands poise and sound judgment. But fear and apprehension can't be the new normal.