The network of agile and semiautonomous cells shows the reach of the extremist group in Europe even as it loses ground in Syria and Iraq.
The officials, including European and Iraqi intelligence officials and a French lawmaker who follows the jihadi networks, described camps in Syria, Iraq and possibly the former Soviet bloc where attackers are trained to target the West. Before being killed in a police raid, the ringleader of the Nov. 13 Paris attacks claimed he had entered Europe in a multinational group of 90 fighters, who scattered "more or less everywhere."
But the biggest break yet in the Paris attacks investigation - the arrest on Friday of fugitive Salah Abdeslam - did not thwart the multipronged attack just four days later on the Belgian capital's airport and subway system that left 31 people dead and an estimated 270 wounded. Three suicide bombers also died.
Just as in Paris, Belgian authorities were searching for at least one fugitive in Tuesday's attacks - this time for a man wearing a white jacket who was seen on airport security footage with the two suicide attackers. The fear is that the man, whose identity Belgian officials say is not known, will follow Abdeslam's path.
After fleeing Paris immediately after the November attacks, Abdeslam forged a new network back in his childhood neighborhood of Molenbeek, long known as a haven for jihadis, and renewed plotting, according to Belgian officials.
"Not only did he drop out of sight, but he did so to organize another attack, with accomplices everywhere. With suicide belts. Two attacks organized just like in Paris. And his arrest, since they knew he was going to talk, it was a response: 'So what if he was arrested? We'll show you that it doesn't change a thing,'" said French Senator Nathalie Goulet, co-head of a commission tracking jihadi networks.
Estimates range from 400 to 600 Islamic State fighters trained specifically for external attacks, according to the officials, including Goulet. Some 5,000 Europeans have gone to Syria.
"The reality is that if we knew exactly how many there were, it wouldn't be happening," she said.
More than four sources with access to tallies of fighters tasked with Europe attacks independently corroborated the numbers of fighters who trained for specific attacks in Europe, including some who have spoken to fighters directly. Others have cross checked information regarding fighters leaving or returning.
Two of the suicide bombers in Tuesday's attacks, Belgian-born brothers Ibrahim and Khalid El Bakraoui, were known to authorities as common criminals, not anti-Western radicals until an apartment one of them rented was traced to Abdeslam last week, according to Belgian state broadcaster RTBF. Similarly, an Algerian killed inside that apartment on March 15 had nothing but a petty theft record in Sweden - but he'd signed up as an Islamic State suicide bomber for the group in 2014 and returned to Europe as part of the Nov. 13 plot.
In claiming responsibility for Tuesday's attack, the Islamic State group described a "secret cell of soldiers" dispatched to Brussels for the purpose. The shadowy cells were confirmed by the EU police agency, Europol, which said in a late January report that intelligence officials believed the group had "developed an external action command trained for special forces-style attacks."
French speakers with links to North Africa, France and Belgium appear to be leading the units and are responsible for developing attack strategies in Europe, said a European security official who spoke on condition of anonymity because he was not authorized to discuss briefing material. He is also familiar with interrogations of former fighters who have returned to Europe. Some were jailed after leaving IS while others were kicked out of the terror group, and they include Muslims and Muslim converts from all across Europe.
Fighters in the units are trained in battleground strategies, explosives, surveillance techniques and counter surveillance, the security official said.
"The difference is that in 2014, some of these IS fighters were only being given a couple weeks of training," he said. "Now the strategy has changed. Special units have been set up. The training is longer. And the objective appears to no longer be killing as many people as possible but rather to have as many terror operations as possible, so the enemy is forced to spend more money or more in manpower."
Similar methods had been developed by al-Qaida but IS has taken it to a new level, he said. Another difference is that fighters are being trained to be their own operators - not necessarily to be beholden to orders from the IS stronghold in Raqqa, Syria, or elsewhere.
Several security officials have said there is growing evidence to suggest the bulk of the training is taking place in Syria, Libya and elsewhere in North Africa.
In the case of Tuesday's attacks, Abdeslam's arrest may have been a trigger for a plot that was already far along.
"To pull off an attack of this sophistication, you need training, planning, materials and a landscape," said Shiraz Maher, a senior research fellow at the International Centre for the Study of Radicalisation at Kings College in London, which has one of the largest databases of fighters and their networks.
"Even if they worked flat out, the attackers in Brussels would have needed at least four days," said Maher, who has conducted extensive interviews with foreign fighters.
The question for many intelligence and security officials is now turning to just how many more fighters have been trained and are ready for more attacks.
A senior Iraqi intelligence official who was not authorized to speak publicly said people from the cell that carried out the Paris attacks are scattered across Germany, Britain, Italy, Denmark and Sweden. Recently, a new group crossed in from Turkey, the official said.
On Wednesday, Turkish authorities said one of the Brussels suicide attackers, Ibrahim El Bakraoui, was caught last June near the Syrian border and deported to the Netherlands, with Ankara warning Dutch and Belgian officials that he was a "foreign terrorist fighter." But he was released from Dutch custody due to lack of evidence of involvement in extremism.
Belgian Justice Minister Koen Geens said Wednesday that authorities had no reason to detain El Bakraoui because he was "not known for terrorist acts but as a common law criminal who was on conditional release."
The latest new name to surface this week, Najim Laachraoui, turned out to be the bombmaker who made the suicide vests used in the Paris attacks, according to French and Belgian officials. Attackers used an explosive known as Triacetone Triperoxide, or TATP, made from common household chemicals. DNA evidence indicates he died on Tuesday in the suicide attack on the airport, two officials briefed on the investigation told AP.
Fifteen kilos of TATP were found in an apartment linked to the Brussels attackers, along with other explosive material.
The unidentified man seen on security footage wearing a white jacket and black hat at the Brussels airport on Tuesday remains at large, a fugitive link in a chain still being forged.
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Dodds reported from London. Qassim Abdul-Zahra contributed from Baghdad.
