America’s unique, enduring gun problem, explained

At least 18 people have been killed and more than a dozen injured in a mass shooting in Lewiston, Maine, the northern state’s second-largest city.

The shooter wasn’t immediately apprehended and appears to have opened fire at two locations on Wednesday evening: a bowling alley and a bar. State and local law enforcement asked residents of Lewiston and the surrounding area to shelter in place as officials worked to find the shooter. Schools and many businesses in the area have been shuttered.

The shooter, whose picture has been shared as...