The Islamic State of Iraq and Greater Syria (ISIS) is gaining notoriety for its barbaric methods, after videos showing beheadings and mass killings surfaced online.
Meanwhile, the group has been attracting an increasing number of foreign fighters from the West, analysts say. But why are so many foreigners joining ISIS's fighting ranks? Among a range of explanations, one of them is that, compared with other jihadist groups like al-Qaeda, ISIS is extremely welcoming to foreigners, says Joshua Landis, a Syria specialist at the University of Oklahoma.
"The biggest reason for that is that ISIS philosophically has welcomed all Muslims as equals, as it’s building an Islamic state which does not have particular Syrian angle," Landis says. "Also, ISIS's leadership is made of people with very prominent roles that are foreigners so you’re not going to be discriminated against philosophically if you're foreign."
Social media also plays a significant role.
While in the past jihadist groups operated in secretive online forums, ISIS spreads its message — both in English and Arabic — on Twitter and Facebook, which are inherently open to the public. With its sleekly produced propaganda videos, ISIS reaches young, restless Muslims or other devotees around the world with a cause that they see is worth fight for, experts say.
"For many people who are lacking a strong sense of identity and purpose, their violent radical global narrative provides easy answers and solutions: it can be very powerful message for people who are looking for answers," says Matthew Levitt, the director of counterterrorism and intelligence at the Washington Institute for Near East Policy. "Their online material shows capturing territory, establishing states, beheading enemies: they show that they are the sexiest jihadi group on the block."
The U.S. State Department estimates that about 12,000 foreigners have traveled to Syria from at least 50 different countries to fight with a number of different groups, including ISIS. Marie Harf, a deputy spokeswoman for the State Department, told CNN that officials estimate the number of Americans fighting with Syrian-based groups ranges from several dozen to 100.
For more on ISIS' recruiting techniques, watch the video above with TIME editor Matt McAllester.