(CBS News) As theentered its third day, the U.S. State Department confirmed that five Americans were wounded in the shopping complex.
CBS News special correspondent John Miller, a former assistant director at the FBI, said on "CBS This Morning" that the unfolding attack sheds new light on the relatively young al Qaeda-linked terrorist group's evolving mission.
Al-Shabab, which means "youth" in Arabic, was established in 2006 as a militant wing of the Islamic fundamentalist forces that controlled parts of Somalia at the time. Its stated mission was to bring Islamic sharia law to Somalia and topple the government.
By 2008, the U.S. had designated it a foreign terrorist group. In 2012, the organization formally allied itself with al Qaeda, issuing a joint announcement by the head of al-Shabab and Ayman al-Zawahri, then al Qaeda's deputy leader. Al-Shabab has provided the larger network significant cash from it's lucrative kidnapping and piracy operations in Somalia in return for weapons.
An unknown number of young Somali-American men, predominantly from areas in the U.S. like Minnesota and Oregon where there are large Somali populations, have been drawn in by the group's propaganda and traveled to eastern Africa to join its ranks.
An al-Shabab spokesman denied reports Monday that Americans were involved in the Nairobi attack, but the FBI is investigating the possibility.Miller notes that in 2010, a U.S. national who had joined al-Shabab was behind a suicide bombing attack on Somali intelligence headquarters.
It was a "very interesting case that took us in the FBI by surprise... that was a real eye-opener for us. It opened the investigation that led us to find that 15 or 20 Americans had found their way from cities from Minneapolis to Portland to join the group," said Miller.
"This starts out with people who are living in a large Somali community in Minneapolis, young people who are hearing the call, [in part due to] internet radicalization...[to] fight for nationalist pride," Miller said.
"When they got there, they found a very different scenario going on," he added, hinting at a growing internal divide over al-Shabab's overall mission among the group's leaders.
Some senior members wanted to keep the group focused domestically, but there is an "entire other faction -- influenced heavily by al Qaeda's headquarters -- that says that the group's mission is global jihad," said Miller.
The attack in Kenya, just across the border, but outside of Somalia, constitutes a direct attack on the foreign economy, explained Miller, and seems to indicate a shift towards global jihad.
Last week, new information surface which also seemed to suggest that al-Shabab is expanding its ambitions. A report filed in a court case in New York's Eastern District alleged that al-Shabab is or was recently operating "a research and development" to develop.