The latest evidence suggests that an inside man at the airport in the Egyptian resort area of Sharm el-Sheikh might have been responsible for planting a bomb on a Russian airliner that caused it to crash in the Sinai Peninsula on October 31, killing all 224 people on board.
The self-proclaimed Islamic State's regional affiliate took responsibility for the attack within hours of the plane crash, but Egyptian officials quickly dismissed the claim. Egyptian President Abdel Fatah al-Sisi even called the suggestion "propaganda" during a state visit he paid to the UK over the weekend.
But on Monday, US intelligence officials confirmed to ABC News that electronic intercepts of the terror group's communications suggest that it was in contact with someone at the airport.
This version of events was also confirmed by US lawmakers who had been briefed on the issue.
"ISIS may have concluded that the best way to defeat airport defenses is not to go through them but to go around them with the help of somebody on the inside," Democratic Congressman Adam Schiff, who received classified information on the investigation, told NBC over the weekend, referring to the Islamic State by an alternative name.
This theory is coherent with views that have been leaked by UK and French intelligence officials who have all strongly hinted over the past week that a bomb brought down the plane.
Mokhtar Awad, a research associate at the Center for American Progress who wrote a comprehensive history of Wilayat Sinai, the Islamic State's affiliate in the peninsula, told VICE News that penetrating the airport is well within the group's capabilities.
Wilayat Sinai began as a local group concentrated on strikes against Israel, but in 2014 its leaders swore an oath to the Islamic State and stepped up their attacks on Egyptian targets. Over the past year the group has waged a guerrilla war against the Egyptian state, successfully bombing major military installations across the country and killing hundreds of security personnel in the Sinai.
"They have been able to infiltrate much harder targets in the past," Awad said.
In 2014, the group blew up Security Directorate buildings in both Cairo and Mansoura with the help of an inside man in the police. Awad noted that operatives of the group who are embedded within the Egyptian military have tipped off the Islamic State to imminent airstrikes on a number of occasions.
"They've been able to turn police officers and some military guys — that's far more difficult than what people are alleging about an airport worker, " he added, though he conceded that building a bomb capable of taking down a plane would represent a new level of sophistication for the group. "When you think through all of the attacks the group has pulled off on it's own so far, you notice they all rely on IEDs [improvised explosive devices]. We haven't seen them using small bombs that would be more effective on a plane."
But the group has it has begun to employ more sophisticated techniques, such as coordinated suicide bombings, since it allied itself with Islamic State a year ago.
"We can't ignore the fact that since pledge of allegiance to ISIS, there has been a transfer of knowledge," Awad said. "A possible scenario is that guys from the Sinai went to Syria and Iraq. They had some time on their hands and no shortage of resources, so they were able to experiment with ways of developing such a bomb."
So far, the official Egyptian-led investigative team has yet to comment on such a scenario. Over the weekend, investigators did confirm that the flight's in-flight recorder captured an abrupt sound consistent with a bomb blast.
"A noise was heard in the last second of the CVR recording," said Ayman al-Muqaddam, head of the investigations committee, but he did not comment on what might have caused it. "A spectral analysis will be carried out by specialized labs in order to identify the nature of this noise."
On Monday, more than a week after the bombing, local investigators announced that they were about to review surveillance tape and employee records at the airport.