The suicide assault on Istanbul's Ataturk airport bears the "hallmark" of an Islamic State group attack, CIA director John Brennan said on Wednesday.
No group has claimed responsibility for Tuesday's attack, which killed 41 people and wounded 239 more.
"The despicable attacks in Istanbul International airport yesterday that killed dozens and injured many more certainly bears the hallmark of ISIL's depravity," Brennan said, using an acronym for the IS group.
Witnesses described scenes of terror and panic as the attackers began shooting indiscriminately and then blew themselves up at the entrance to Ataturk airport, one of Europe's busiest hubs.
The attack followed coordinated IS group suicide bombings in March at the international airport and a metro station in Brussels that left 32 people dead.
The IS group typically does not claim responsibility for attacks in Turkey in order to send messages to the government without alienating potential future recruits in the country, Brennan said.
The IS group may well be planning a similar attack in the United States, America's top spy added.
"If anyone believes that the US homeland is hermetically sealed... I would guard against that," he said.
Earlier this month, a gunman killed 49 people at a Florida gay nightclub in the deadliest terror attack on US soil since September 11, 2001.
The shooter, US-born Omar Mateen, pledged allegiance to the IS group while carrying out the attack.
Speaking in Washington at the council on foreign relations think-tank, Brennan opened his remarks by saying the intelligence-sharing partnership between Britain and the United States would not be affected by the United Kingdom's decision to leave the European Union.
"The bonds of friendship and cooperation between (US and UK spy) services are only destined to grow stronger in the years ahead," Brennan said.
Still, he noted, Britain's momentous decision may pose the greatest challenge the European bloc has ever faced.