CAIRO • The Islamic State in Iraq and Syria (ISIS) branch suspected of bringing down a Russian airliner in Egypt had eluded a security dragnet by operating in secretive cells inspired by a leader who used to import clothes for a living, Egyptian intelligence officials say.
Western officials are increasingly pointing the finger of blame at the group called Sinai Province, which has focused on killing Egyptian soldiers and police since the military toppled president Mohamed Mursi of the Muslim Brotherhood in 2013 after mass protests.
If solid evidence emerges it attacked the aircraft, that would instantly propel the group to the top of the terrorism ladder, with one of the deadliest attacks since Al-Qaeda flew planes into the World Trade Center in New York in 2001.
If a bomb knocked the Airbus A321 out of the sky, that would challenge Egypt's assertions that it had brought under control militants who have carried out high-profile attacks on senior government officials and Western targets.
Security experts and investigators have said the plane is unlikely to have been struck from the outside and Sinai militants are not believed to have any missiles capable of striking a jet at 30,000 feet.
Sinai Province is partly the product of Egypt's efforts to eliminate militancy, which has threatened the most populous Arab country for decades, according to the intelligence sources.
The three officials, who closely follow the Sinai-based insurgency, say many of its fighters fled to Syria after Mursi was removed and then army chief Abdel Fattah al-Sisi unleashed security forces on Islamists, both moderate and radical.
Sinai Province's leader - a 42-year-old former clothes importer known by his nom de guerre Abu Osama al-Masri - studied at Al-Azhar, a 1,000-year old Egyptian centre for Islamic learning that supports the government, said the officials.
But like others who studied in a centre known for its moderation, he was radicalised and took up arms in Sinai before heading to Syria with about 20 followers when security forces clamped down on Islamists after Mursi's departure, the sources said.
There, he and the other fighters gained experience that would prove useful upon their eventual return to the Sinai Province, when they were approached by ISIS and embraced its goal of creating a caliphate across the Muslim world.
It seems they were mesmerised by ISIS' mysterious Iraqi leader, Abu Bakr al-Baghdadi, said the officials.
ISIS sent arms and cash by boat from Iraq to neighbouring Libya, where militants have thrived in the chaos that followed the fall of Muammar Gaddafi in 2011, said another intelligence official.
A porous border then enabled Baghdadi's supporters to travel to Sinai, on the other side of Egypt, to deliver the goods to Islamist militant comrades, the officials added.
"Other militants taught them how to evade capture and they learnt how to shoot accurately and assemble bombs," said one of the intelligence officials, speaking on condition of anonymity.
"They became experts."
Sinai Province now consists of only hundreds of militants scattered into groups of five to seven men, which have few links to reduce the chances of capture, said the officials. "They are very secretive," one of the intelligence officials said. "Each cell doesn't know about other cells."
A tribal leader in the Sinai Province said he had recently noticed pro-ISIS militants driving around in new Toyota Land Cruisers. Some had Apple computers.
"It seems they are getting more and more ambitious," he said.