Sari Horwitz

As their photographs spread rapidly across the Internet, the Tsarnaev brothers decided to make their move. Not waiting for police to find them, they gathered guns and homemade explosives for what became a final, bloody rampage on a community still in shock from the bombings three days before.

In less than 15 minutes late Thursday, authorities said, the brothers fatally shot a campus police officer as he sat in his car, then carjacked a Mercedes-Benz sport-utility vehicle at gunpoint. They held the driver hostage for 30 minutes as they scoured Boston’s western suburbs for bank machines from which to take the man’s cash.

WASHINGTON — “Are the president’s kids more important than yours?” the deep and dramatic-sounding voice intoned. “Then why is he skeptical about putting armed security in our schools when his kids are protected by armed guards at their school?”

When the National Rifle Association aired its 35-second TV spot last month, suggesting that President Barack Obama has a double standard on school security and seemingly using his daughters as props, the White House quickly labeled it “repugnant and cowardly.” But the commercial was another in a long line of bare-knuckled NRA advertisements, many of them controversial but also compelling attacks that have come to define the organization.

LAS VEGAS _The tens of thousands of gun enthusiasts, sellers and manufacturers gathering here for the world’s largest firearms show are well aware of the political battle unfolding in Washington over tighter controls on assault-style rifles and high-capacity magazines. They are neither worried nor reassured.