At least 400 Islamic State in Iraq and Syria (ISIS) fighters have been trained and sent to Europe to carry out attacks like the ones in Paris and Brussels, according to an Associated Press report Wednesday.

The fighters are organized into semiautonomous cells that are empowered to choose when, where and how they attack, according to the report, which cited European and Iraqi intelligence officials and a French lawmaker.

Tuesday’s attack in Brussels, which struck an airport and a metro station, killed more than 30 people and injured upward of 200 others.

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In claiming responsibility for the attack, ISIS said it had a "secret cell of soldiers" dispatched to Brussels.

Estimates place the number of ISIS fighters in Europe at anywhere from 400 to 600, according to the report.

The fighters were trained in camps in Syria, Iraq and possibly the former Soviet bloc, according to the report. They have been trained in battleground strategies, explosives, surveillance techniques and counter surveillance.

The strategy is similar to al Qaeda’s, but the ISIS fighters do not have to follow specific orders from ISIS’s de facto capital in Raqqa, Syria.

The European cells appear to be led by French speakers with links to North Africa, France and Belgium. Their objective appears to be to conduct as many terrorism operations as possible, an anonymous European security official told the AP.

"The difference is that in 2014, some of these IS fighters were only being given a couple weeks of training," he said to the AP. "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. It's more about the rhythm of terror operations now."

Salah Abdeslam, the suspected Paris attacker arrested last week, appears to have gone from fleeing Paris to forming a new network in his childhood neighborhood of Molenbeek, the report added.

"Not only did he drop out of sight, but he did so to organize another attack, with accomplices everywhere,” said French Senator Nathalie Goulet, co-head of a commission tracking jihadi networks, told the AP. “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.'"

The exact number of ISIS fighters in Europe is unknown, she added.

"The reality is that if we knew exactly how many there were,” she said, “it wouldn't be happening.”