Pumwani, Kenya (CNN) -- Asha Mohamed sits in her cramped room in Pumwani slum clutching a tiny photo of her son, Harun. He's dressed in a blue-striped tie framed by a crisp white T-shirt -- a typical 15-year-old Kenyan high school student.
But in September he vanished. "Harun woke up very early and asked his sister "what time is it?" says Asha. He kept on asking her again and again. Then, at four in the morning, he left the house."
In her heart, Asha knew where he had gone, but the text messages later confirmed it. Harun left his school and home in Kenya to fight for al Qaeda-linked Al Shabaab.
"It started when he was 14. He came to me many times and said "mom, I am going to Somalia to fight Jihad." I thought he was just playing."
For years, Al Shabaab has targeted Somalis abroad to fight in their campaign to overthrown the weak transitional government. Now Kenyans, with no ethnic link to Somalia, are joining the Jihad.
According to a recent U.N. report, there are "extensive Kenyan networks linked to Al-Shabaab, which not only recruit and raise funds for the organization, but also conduct orientation and training events."
Many of those events centered on Pumwani, a largely Muslim slum in Nairobi. Here, residents and religious leaders speak of a charismatic young Kenyan Sheikh that arrived from Mombasa.
They say he bravely stood up to corruption, promoted the Quran, and generously handed out scholarships to young men. And he stoked their passion for Al Shabaab.
The man is Sheikh Ahmad Iman Ali -- now the leader of Al Shabaab's Kenya cell.
He is remembered fondly at the Maratib Islamic Center in Pumwani.
"He was fearless and, at the same time, kind," says Abdullah Kilume, the administrator of the center. "The majority think he was a good man, he did a lot of good, they saw what he did."
Sheikh Iman would conduct lengthy sermons at the center's mosque, says the U.N. report. The sermons were called 'Jihad Training sessions' and many of them are posted on the internet.
According to the U.N's investigations, training sessions included classroom based lectures about 'Islamic Struggles' in Afghanistan, Iraq and Yemen. Many of the children wore "Jihad is our Religion" T-shirts.
Kilume remembers Sheikh Iman's teachings well. He said that he appealed to the Kenyan youth of Pumwani. "It reached the youth. I personally know that some of them wanted to go to Somalia."
But while parents in this area supported the Sheikh's war on graft and welcomed his financial help in this grindingly poor place, they felt differently when their children started to vanish.
"I went to that mosque and caused havoc, but they didn't care," says Asha. "Somalia, that is not his war, I don't even know why they are fighting, so why should he leave here and go fight in something he doesn't even understand."
But increasingly, it seems, these Kenyan youth do see it as their fight too. Six of Harun's school friends who attended the center also vanished, she says.
Local authorities wouldn't comment specifically on the case, but they acknowledged that a problem exists.
The youth, who fled face the prospect of fighting Kenyan soldiers, many of them Muslim, who are engaged in an incursion deep into Somalia to stamp out Al Shabaab.
Kilumi, the Muslim Center administrator, says he can accept it if these Kenyan recruits to Al Shaabab end up killing Kenyan soldiers.
"Like every other Muslim, I would like the Sharia law to be implemented and that is what they want to do in that part of Somalia," he says. "Muslims first, Kenyans second."
Sheikh Iman's recruiting and fundraising drive has apparently been so successful that he has been rapidly promoted through the ranks of Al Shabaab.
In 2009 he moved his base to Somalia, where it's believed that he commands a force of between 300 and 500 Kenyan fighters.
In January a video of the sheikh was posted on jihadi websites.
It's not known when the video was recorded. It shows him seated cross-legged in battle fatigues. Sheikh Iman called again for Kenya's youth to cross over the border to fight. And if they couldn¹t, he gave them a simple message.
"Raise your sword against the enemy that is closest to you. Jihad should now be waged inside Kenya."