In a rare public speech, Britain’s MI5 Director General Andrew Parker warned that al-Qaeda fighters in Syria are planning to send jihadists back to the West to inflict “mass casualties” on targets in multiple nations.

The speech had been planned for awhile, but the warning seems much greater coming just a day after an attack by a pair of gunmen, themselves said to be recent returnees from Syria, killed 12 at a magazine in Paris.

The two returnees were just two of many. France alone has 1,000 people it is tracking, and the US Justice Department says that some 13,000 foreign fighters are in Syria and Iraq, with over 3,000 from Western Europe alone.

If these two brothers were able to carry out such a significant attack by themselves, the number and scope of the attacks that these returnees could carry out if they returned en masse is absolutely staggering.

These estimates for how many fighters from different countries have come to Iraq and Syria are always just rough estimates, and a lot of the people aren’t known by name. The Kouachi brothers, the suspects in this case, were known to officials for many years, and that still wasn’t enough to prevent the attack.

As the ISIS war escalates, their ability to recruit seems to be growing greatly, and the number of potential returnees is only growing, even as intelligence communities struggle to get a handle on the number already there.