Two deadly attacks that killed at least 91 people Friday ushered in a black weekend for Norwegians, and is a sad reminder that home-grown extremism remains a grave threat across the world.

Few people would have expected this "national tragedy", as Norwegian Prime Minister Jens Stoltenberg put it, to hit the wealthy and traditionally peaceful Nordic country.

Preliminary investigations indicate the suspect, a 32-year-old Norwegian man, has nothing to do with international terrorist groups, while local media said the horrific carnage, which deprived many families of their young children, appears to be the crazy act of a local extremist who wanted revenge on society.

People place flowers and light candles for condolence in Oslo, capital of Norway, July 23, 2011. At least 84 people were killed when a gunman disguised as a police officer opened fire at a youth camp on Utoeya island, about 40 km west of Oslo. The shootings occurred hours after a massive explosion ripped through a high-rise building housing the prime minister's office in Oslo, killing seven people. [Xinhua]

Although the real motivation of the attacks remains unknown, investigators have already identified some traits of extremism, such as extreme cruelty, anti-humanism and anti-rationality, in the attacks.

The gunman, dressing as a police officer, went on a shooting rampage at the ruling Labor Party's youth camp on the island of Utoya, some 40 kilometers west of Oslo, killing at least 84 people, hours after a bomb that he allegedly planted near government offices in downtown Oslo went off and killed at least seven.

Witnesses on the island said the gunmen did not even spare the young people who had jumped into the sea in a desperate bid to escape.

It would have been unimaginable moments before the rampage that the young people enjoying their day at the camp would die so suddenly and violently. It's also hard to imagine how their parents can bear the pain of their loss.

Violent attacks by lone extremists, while not rare in history, are in most cases more difficult to trace and prevent than those organized by terrorist groups.

In recent years, some Western countries have experienced random tragedies, labelled "campus shootings", "serial killings" and other terms, which were often masterminded by a rogue extremist.

The truck bombing at a federal building in Oklahoma city in 1995 that killed 168 people was carried out by a U.S. Army veteran who held grudges against the federal government.

With the development of modern technology and advances in weaponry, terrorism is becoming more deadly. Additionally, compared with organized terrorist attacks, terrorism that originates from lone extremists is in most cases harder to predict and prevent, making it more likely to lead to mass casualty. Therefore, governments and citizens around the world have a duty to firmly resist, eliminate, prevent and fight this kind of extremism.

With further investigation underway, the murderer who caused this bloodshed will surely receive the punishment he deserves. But the wound on the heart of every Norwegian caused by the tragedy won't heal any time soon. As well as sharing Norway's pain in resonance, the world should also ponder how to better prevent these kinds of acts, how to never again let dark days like this occur.