The deaths of more than 70 people, many of them teenagers at a summer camp, shocked the people of Norway and the world. A gunman with extremist political views is in custody, but that fact does not render comprehensible the violence.

The suspect, Anders Behring Breivik, argued in a bizarre Internet manifesto that he was forced to act to help save Europe from multiculturalism.

His targets at the shooting site were teenage members of the social-democratic Labor Party. Breivik himself, 32, had left the more conservative of Norway’s political parties because he felt it insufficiently anti-immigrant.

As Americans after the Oklahoma City bombing of 1995 can attest, home-grown terrorism is in a way the worst.

President Barack Obama appropriately expressed the sympathy of the people of the United States for the victims and for Norway, our friend and ally.

The worst violence in Norway since that country’s valiant resistance to the Nazis in World War II is a shocking reminder that vigilance against terrorism remains necessary in today’s world.