ISTANBUL -- Investigators are piecing together the clues, hoping to identify the Istanbul airport attackers -- and who sent them.

The killers -- who were armed with automatic weapons, grenades and suicide vests --arrived in a cab together and split up.

Those tactics alone -- according to U.S. intelligence officials -- suggest that the Islamic State of Iraq and Syria (ISIS) was behind it. CIA Director John Brennan did not officially assign blame on Tuesday, but he came close.

"The despicable attacks at Istanbul's national airport yesterday that killed dozens and injured many more certainly bears the hallmarks of ISIL's depravity," he said, using an alternate acronym for the group.

CBS News has learned that there had been ISIS "chatter" in recent months indicating that the group was intent on targeting transportation hubs in Turkey -- including this airport.

A U.S. intelligence source said Turkish investigators have located the taxi driver who dropped the attackers off at the airport. When he was interviewed, the driver said he didn't recognize the language the men were speaking.

The working theory is that the attackers may have been Chechen jihadists from southwest Russia, who could have links to ISIS, reports CBS News correspondent Margarat Brennan, who is traveling with President Obama in Ottawa. DNA tests are underway, and the U.S. doesn't want any conclusion to be influenced by Turkey's own domestic terrorism and politics.

The day before the attack, the U.S. State Department updated a Turkey travel warning, which alerted U.S. citizens to "increased threats from terrorist groups throughout the country."

CBS News has learned that there had been ISIS "chatter" in recent months indicating that the group was intent on targeting transportation hubs in Turkey -- including this airport.

If this is ISIS again, CBS News terrorism consultant Richard Walton said that it would signal a vulnerability that is extremely difficult to defend.

"Obviously it's the second [attack] against a major airport in three months following the attack at Brussels airport," he said. "And it will make security professionals around the world concerned, gravely concerned, about the security of airports."

After the attacks last year in Paris, intelligence officials in Europe warned that ISIS leaders had trained cells of operatives for coordinated international attacks.

The CIA director said Tuesday that he would be surprised if ISIS is not trying to carry out the same type of attack in the U.S. And he added that it is difficult to stop someone willing to die.