While exact numbers are elusive, ISIL has at least tens of thousands of fighters, most of whom are battle-tested, including thousands of foreign fighters. That is a far larger force of potential terrorists than Al Qaeda possessed when it carried out 9/11 (likely well under 1,000). ISIL can easily staff a spectacular attack, in fact, it has had that capability for well over a year.
ISIL also has money. Again, specific and credible estimates are wanting, but based on its estimated peak wealth in 2014, the group today is believed to have tens of millions of dollars in reserve, and more likely hundreds of millions. In comparison, Al Qaeda incurred about $500,000 in direct costs for the 9/11 attacks, so funding a spectacular attack is also within ISIL’s reach, and has been for well over a year.