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Flash Points: The growing threat of al-Shabaab in Kenya

This week's bloody slaughter of nearly 150 people at a Kenyan college has the world on edge -- and it's just the sort of fear terrorist group al-Shabaab wants to inspire.

"This demonstrates that al-Shabaab is still alive," CBS News senior national security analyst Juan Zarate said of the Somalia-based extremists. "It's kicking. And it's dangerous."

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The terrorist organization, a known al Qaeda affiliate, has promised that the massacre at Garissa University College will not be its last in the Kenya. The Islamic militants released public statements saying "Kenyan cities will run red with blood" in revenge for killings carried out by Kenyan troops fighting the rebel group in Somalia.

"The Kenyans are having a hard time, I think, defending their borders and certainly defending infiltrations by al-Shabaab cells," Zarate said, indicating that this could be the start of a "worrisome trend."

The group first came to international attention after they laid claim to the September 2013 hostage situation at an upscale shopping mall in Nairobi, Kenya. The 30-hour siege at the Westgate Mall served as an inspiration for al-Shabaab's recent threat against Minnesota's Mall of America in February. And though U.S. intelligence officials assessed the threat levels skeptically, al-Shabaab's video calling for the "lone wolf"-style attacks was enough to prompt major shopping centers to take extra security precautions.

"This most recent release is emblematic of a new phase that we're in," Homeland Security Secretary Jeh Johnson told CBS News in the wake of those threats. "And so doing things here in the homeland has become critical to deal with this new global terrorist threat we face."

This concern, voiced in February by Johnson, is resurfacing with recent upticks in homegrown extremist plots.

Zarate points to al-Shabaab's probable "access to westerners" as a major concern, citing reports of Americans, Australians and other Western Europeans who have been lured by jihadist ideologies.

"The real question and danger is: Are they able to assemble even just a handful of these characters and send them back to the West?" Zarate said.

The strategic impact of the group's "global Jihadi DNA" can be felt even without an assault that reaches the magnitude of the September 11th attacks. The al Qaeda cell's ideological ripple effect, Zarate said, can be just as great with attacks on education sites or attacks on Christians -- any of these can "foment additional tension and terror" in Western allies.