Abdullahi, then aged about 20, initially dismissed those men as opportunists who had pretended to convert to Islam to win work as guns for hire.
Then he saw them in battle.
"They were good fighters. I saw the way they would advise us to fight, to defend ourselves," Abdullahi said of his two years in al Shabaab, during which time he fought Somalia's weak United Nations-backed government. "I fought one battle outside Mogadishu. Half of us died... (The Kenyans) were very brave, the way they ran towards gunfire."
That's exactly what worries Kenyan and Western security agencies. Al Shabaab has been waging an insurgency against Somalia's fragile interim government since 2007 and formally became part of al Qaeda earlier this year. Abdullahi's account is part of a mounting body of evidence - including intelligence picked up by security agencies, research by the United Nations and accounts by Muslim Kenyans interviewed for this story - that suggests al Shabaab is mentoring a new and increasingly multi-ethnic generation of militants in the region.
That could have major ramifications not just for Somalia, which has been without a working government for two decades, but also for countries such as Kenya, Tanzania and Uganda, relatively stable democracies whose economies are among the steadiest in Africa. This week, Kenyan politicians blamed a bombing in central Nairobi on al Shabaab, which means "Youth" or "Boys" in Arabic.
Al Shabaab seeks to impose a strict version of Sharia or Islamic law. The group emerged as a force in 2006 as part of a movement that pushed U.S.-backed warlords out of Somalia's capital, Mogadishu. It remains Somalia's most powerful non-government armed group and in its propaganda, promotes the idea that many Muslims are flocking to its cause around Africa.
Washington and London have long worried the Somali group aimed to expand its influence in Africa. That suspicion was confirmed last July when a United Nations investigation found al Shabaab had created extensive funding, recruiting and training networks in Kenya.