American intelligence officials have so far characterized the Sri Lanka attacks as having been inspired by the Islamic State, as opposed to having been executed directly by the group. But analysts say there may be some middle ground.

A video released by ISIS on Tuesday, showing members of National Thowheeth Jama’ath pledging fealty to Mr. al-Baghdadi, shows at the least that the group had a means of communicating with core ISIS operatives and was able to transmit video to them.

“The fact that the attackers knew the right people in ISIS to send the video to so that it would be released through its official media channel, shows that it’s more than mere inspiration,” Amarnath Amarasingam, an expert at the Institute for Strategic Dialogue, said on Twitter on Wednesday. “That’s just one of many pieces of info emerging pointing to a more directed attack.”

So far, there is no public evidence that ISIS played an active role in guiding or otherwise aiding the Sri Lanka attack.

This gray area may be a fertile environment for the group’s future.

The attack was among the deadliest ever carried out by Islamic State acolytes outside Iraq and Syria. That the group responsible for it existed so far below the radar of international intelligence agencies troubles counterterrorism officials, who wonder how many similar groups are active or operating surreptitiously in North Africa, South Asia and elsewhere.

Current and former counterterrorism officials warned that the Sri Lanka bombings may be a harbinger for a new phase of ISIS attacks.

“Former ISIS fighters and sympathizers are rebranding themselves ideologically with other terrorists,” said Christopher P. Costa, who was a senior director for counterterrorism on the National Security Council under the Trump administration. “It’s not just a question of the loss of a physical caliphate so much as considering exactly what ISIS will look like as it tries to reconstitute itself.”

“We will see more of these kinds of attacks in the future,” he said.