In New Zealand, Signs Point to a Gunman Steeped in Internet Trolling

A camera mounted to his head, the gunman who livestreamed part of his savage attack on two mosques in Christchurch, New Zealand, on Friday began his video by casually making reference to a current internet meme.

He appeared to be steeped in the culture of the extreme-right internet. And in the terrible minutes of video that followed, he proved to be a nonchalant, unrepentant killer.

As of Friday night, the gunman had not been identified by the authorities. But just before the attack began, a man who said he was...