The terrorists who killed 42 and injured more than 200 at the Ankara airport were from Russia, Uzbekistan and Kyrgyzstan, according to Turkish authorities.

A Turkish source told CNN that the Islamic State leaders were directly involved in planning the attack, and that the terrorists entered the country about a month ago:

Officials believe the men -- identified by another Turkish official and state media as being from Russia, Uzbekistan and Kyrgyzstan -- entered Turkey about a month ago from Raqqa, along with the suicide vests and bombs used in the attack, the source said.

They rented an apartment in the Fatih district of Istanbul, where one of the attackers left behind his passport, the Turkish government source told CNN.

The attack was "extremely well planned with ISIS leadership involved," the source said.

While no one has yet claimed responsibility for the airport assault, CNN contributor Michael Weiss, author of "ISIS: Inside the Army of Terror," said the nationalities revealed Thursday buttress the claim of ISIS involvement.

"One of the toughest battalions in ISIS is called the Uzbek battalion," he said. "These were the guys who were essentially on the front lines guarding Falluja, the city they just lost in Iraq."

"Ask anybody inside ISIS or who's fought ISIS. People from the former Soviet Union tend to be the most battle-hardened and willing to die," he said.

Also Thursday, authorities detained 22 people in connection with the attack, according to a Turkish official.

Thirteen people were taken into custody in Istanbul and nine in the coastal city of Izmir, the official said. Three of those detained were foreign nationals, state media reported.

Until recently, Turkey was far more concerned about attacking Kurdish fighters than ISIS terrorists. But after an alarming increase in bombings in Ankara and elsewhere in the country, the Turkish government has altered its strategy to join the coalition fighting ISIS.

Are they too late in taking the ISIS threat seriously? It seems clear that there are ISIS networks already established in Turkey with an unknown number of active cells plotting attacks. Rooting them out will be difficult given that they aren't usually revealed until an attack takes place. 

Despite ISIS losing ground in both Syria and Iraq, their terror operations look to be thriving. It suggests that even if Iraq and Syria can manage to evict them from their territory, the threat of terrorism will not be diminished.