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(CNN) – The militant group Islamic State of Iraq and Syria, or ISIS, is the most dangerous terrorist group right now for three main reasons, an intelligence source tells CNN.
1. ISIS has the most operatives and sympathizers with direct access to U.S. and Western Europe
These are individuals with American or Western passports, who can easily gain entry to the U.S. and Western countries.
"It's what we called back when I was at the agency 'clean skins,' people who can pass through transit points because they have documents that aren't stamped by things that you would find remarkable if you're a security officer at a border," said CNN counterterrorism analyst and former FBI and CIA official Phil Mudd.
2. ISIS has a safe haven
Unlike al Qaeda in Yemen or Pakistan, ISIS has a large swath of territory in Syria and Iraq, giving it a big advantage.
A safe haven gives terrorist groups the space "to plot attacks against Europe or the United States without worrying as much about fighting government forces," said Mudd, who estimates that ISIS is "halfway to safe haven," and more so in Syria.
"We've seen the threat diminish in Yemen and Somalia. Those terrorists groups are running from security services forces. Less so potentially in Syria and Iraq, where we have the safe haven of the future," Mudd said.
ISIS may also be working with al Qaeda groups in Yemen, who have very skilled bomb makers.
"The bomb makers are the ones to focus on," says Mudd.
3. ISIS may be the best-armed and best-funded terror group in the world
The militant group's strategy for generating resources is to launch large-scale attacks aimed at capturing and holding territory. The group overran Mosul, Iraq's second-largest city earlier this month, seizing $400 million from Mosul banks, according to Ayham Kamel, Middle East and North Africa director at Eurasia Group.
ISIS will "use cash reserves from Mosul's banks, military equipment from seized military and police bases and the release of 2,500 fighters from local jails to bolster its military and financial capability," says Kamel.