Over the years, al-Qaida has recruited young men in the Arab world, Africa and Central Asia, including Afghanistan. The group has also had some success in luring followers from Turkey.
Last month, Fahrettin Gumus, a retired security guard from Turkey's northwestern province of Bursa, went to Afghanistan in search of his son, who he had last heard from three years earlier.
The small-framed 57-year old says he often worried about his son Ibrahim, but he never through he'd go through with his plan to join al-Qaida.
"My son is very emotional," Gumus says. "We tried to stop him from getting involved in al-Qaida. He promised us not to commit."
Gumus says that there were several factors that influenced Ibrahim, who was 16 at the time he ran away. The first was his older brother.
"My other son, Veral, came to Afghanistan twice in 2007 and 2008, and I am pretty sure he was involved in al-Qaida," Gumus says. He noted that Veral has now returned to Turkey.
Ibrahim was also swayed by his friends, some of whom were sympathetic to al-Qaida. The third factor was the community in general. Gumus says that Bursa province is a very religious place and there are what he describes as "hubs of radicalism" there. Turkish authorities have arrested a number of al-Qaida suspects in Bursa in recent years.
A Note, A Call And Nothing More
In late 2009, Ibrahim was working in an Internet cafe. One night, he didn't come home.
"He left a note saying he was going off with his friends, and everything is OK," Gumus says. "We found out that he went to Istanbul, and then traveled to Afghanistan."
Four months later, Ibrahim called home. He said he was in Afghanistan's southern province of Helmand, and everything was fine. He said he'd be home soon.
That was the last the family heard from him.
"Both the anti-terrorism department and the police investigated. I waited for months, and they didn't provide any information," Gumus recalls. "Finally, I tried the Ministry of Foreign Affairs, and they couldn't help either."
A Father's Mission
So nearly three years after the last contact from his son, Gumus flew to Afghanistan. He started by going to the Turkish Embassy, where he was told that there was no sign of Ibrahim in Afghanistan.
But the determined father — with no leads or contacts — persevered, and took a bus to the southern city of Kandahar, the birthplace of the Taliban and once a major base for al-Qaida.
"I didn't care that people told me it was dangerous. I'm not afraid of anything. I'm looking for my son," Gumus says.
But, he says, he found nothing there either. People told him that it's possible that Ibrahim had been moved to Pakistan or another country where al-Qaida operates.
He says he never spoke to anyone from al-Qaida. Even though he doesn't know what his next move is, Gumus says he will pursue this until the end - even if it costs his life. He says he has to honor a promise he made to his daughter. He begins to cry when talking about her.
"She feels very sad. She's very sensitive because she's pregnant. She's the biggest reason I left home to find my son," he says. "I promised her I will find her brother and bring him home."
Gumus, who recently returned to Turkey, still clings to the belief that Ibrahim is alive, somewhere. The father says he just wants to send his son a message: "You have made a mistake, but you can return home and make your family happy."
NPR's Sultan Faizy contributed to this report.