Dutch journalist Arjen Westra, 43, was sitting in a café, thinking about his next article, when all hell broke loose. At 11:45 p.m. last Saturday, he suddenly heard a massive explosion and the abrupt retort of assault weapons. It was very near. The café was part of the Westgate Mall in the Kenyan capital Nairobi. The patrons on the terrace panicked and ran inside screaming.

"I felt like I was in a war film," Westra says. "But I sensed right away that this wasn't fiction, but that it was real horror."

Westra somehow managed to flee in the midst of the chaos, just as the killing began inside the mall. Dozens of people were ripped apart by hand grenades or shot to death. For many, the tragedy went on for hours and hours. The attackers had taken hostages, and when Kenyan soldiers arrived at the scene, a bloody battle ensued between the shelves and freezers.

The assault on the shopping center is the worst terrorist attack in Kenya  since the bombing of the United States Embassy 15 years ago. And even as the drama unfolded, the Somali Islamist organization al-Shabaab claimed responsibility for the attack.

Kenyan troops had invaded war-torn Somalia, which borders Kenya, in 2011 to drive back the Islamists , and now they were exacting revenge. While al-Shabaab isn't nearly as powerful in Somalia as it once was, terrorism  experts fear that the organization can now be expected to stage attacks abroad.

Freedom Fighters Against 'Foreign Invaders'

Al-Shabaab means "The Youth" in Arabic. The terrorist group was founded in 2006 as a militant wing of the Islamic Courts Union, a radical group of Sharia courts that had assumed power in southern Somalia and had also gained control of the capital Mogadishu. The young militants saw themselves as freedom fighters against "foreign invaders" from Ethiopia who, with American military assistance, where trying to drive out the fundamentalists.

After a coup in 1991, Somalia had ceased to exist as a nation. The leaderless country, torn apart by conflicts among rival clans, developed into an ideal haven for militant Islamists from around the world -- and al-Shabaab became a melting pot for international Muslim terrorists.

According to US intelligence agencies, mujahedeen from Afghanistan and Pakistan joined al-Shabaab  after the death of al-Qaida leader Osama bin Laden. In February 2012 the organization's leader, Ahmed Abdi Godane, swore allegiance to al-Qaida. Apparently, Somali expatriates from the United States and other Western countries have also joined the group since then.

Al-Shabaab, with an estimated 5,000 militants, also staged attacks abroad from the very beginning. In July 2010, suicide bombers killed 74 people who were watching a television broadcast of the soccer World Cup final in the Ugandan capital Kampala. It was an act of revenge against the Ugandan army, which has been in action in Somalia as part of the African Union Mission in Somalia (AMISOM) peacekeeping force since early 2007.

Kenya Declared a Target for Attacks

When Kenyan troops marched into Somalia and helped drive the extremists out of various areas, including Mogadishu and the port city of Kismayo, the neighboring country was declared a target for attacks. Al-Shabaab still has enough funding to launch operations at home and abroad, partly through donations it receives from Somali expatriates. Most of all, however, the Islamists mercilessly extort money from residents of the areas of Somalia they still control.

Al-Shabaab is absolutely capable of destabilizing the region surrounding the Horn of Africa , warn United Nations security experts. They say that the group's command structures are intact, and that it is hoarding weapons to prepare for the period after the peace mission ends. According to a report prepared for the UN Security Council, al-Shabaab is responsible for hundreds of murders and bombings in recent years.

Annette Weber, an Africa expert with the German Institute for International and Security Affairs, doubts that orders for the latest bloody attack came from Somalia. "The terrorist group's international wing has become stronger," says Weber.

It is clear that the jihadists are increasingly selecting unprotected targets like the Westgate Mall, which can be attacked with relatively little risk. "We will not negotiate with the Kenyan government as long as its forces are invading our country, so reap the bitter fruits of your harvest," an al-Shabaab representative tweeted as the attack was unfolding.

The Islamists' main goal is to inflict as much damage as possible on countries like Kenya  and Uganda, which support Somalia's government, says Daveed Gartenstein-Ross of the Foundation for Defense of Democracies in Washington, adding that they achieved their objective in Nairobi. "With only a few militants, they caused a standoff for several days and left behind enormous devastation."