A powerful suicide blast in Somalia's capital, Mogadishu, on Tuesday by the Islamist group Al Shabab has killed at least 65 people, and once again shaken what little confidence Somalis have in their transitional government.

The truck bomb, driven by two suicide bombers, struck a building compound housing government employees, and killed scores of college students queuing up for results of a scholarship program that would have allowed many Somali students to study in Turkey.

On an Islamist website, Al Shabab took credit for the attack, saying: “Our mujahideen fighters have entered a place where ministers and AMISOM foreigners stay.” AMISOM is the African Union Mission in Somalia, a peacekeeping force composed of some 8,000 Ugandan and Burundian troops with 3,000 more on the way.

The blast marks a turning point of sorts in the ongoing efforts to restore stability to Somalia, a country that has been racked by more than 20 years of civil war since the fall of the government of socialist President Siad Barre in 1991.

After years of fighting Somalia's weak Transitional Federal Government (TFG) and AMISOM in the streets of Mogadishu, Al Shabab pulled out of the capital in early August, announcing a tactical retreat and a promise to return with less conventional methods of war.

Today's attack – the group's most deadly suicide bombing to date – shows that Al Shabab can still strike in the city at will. The blast also serves as a reminder of the weakness of the TFG, says E.J. Hogendoorn, a Somalia expert at the International Crisis Group in Washington.

“The unfortunate reality is that this was to be expected,” says Mr. Hogendoorn. “This was announced by Al Shabab when it withdrew from most of Mogadishu. Essentially, they could not fight AMISOM or the TFG conventionally, so they would adopt asymmetrical warfare tactics such as suicide bombings.”

But while expected, Hogendoorn says, “this raises questions of the capability of the TFG, and it also raises questions of the capability of the AMISOM to protect the areas under their control.”

As AMISOM forces establish their presence further out into Mogadishu, and as the TFG begins to administer more parts of the country, Hogendoorn says, “inevitably this will increase the risks that its security forces take. They are more exposed to attack.”

The United Nations, which also offers diplomatic support to the transitional government but no military peacekeepers, condemned today's attack.

“I am deeply saddened by this senseless and cowardly attack," said Augustine Mahiga, the UN special representative in Somalia. “These actions are

unacceptable. The murder of ordinary Somalis can not be justified for any reason.”

Today’s suicide bombing is not Al Shabab's first.

In late 2009, a suicide bomber killed 24 at a graduation ceremony for doctors and health practitioners in the tiny government-controlled sector of Mogadishu. Shabab has also managed to carry out suicide bomb attacks far from its territory. On July 10, 2010, twin suicide bomb blasts killed some 76 people at two separate restaurants in Kampala, Uganda, while patrons were watching the 2010 World Cup soccer tournament. Al Shabab took credit for those attacks, saying it was punishment for Ugandan participation in the AMISOM peacekeeping force.