At least 22 radical Islamists from Europe linked to the terror network behind the Brussels and Paris attacks are suspected to be still at large, putting security services on high alert as they rush to prevent the Islamic State from striking again in the region.

Many of the fugitives have been involved in previous Islamic State plots, officials say, and almost all of them have spent months or years fighting in Syria.

Interviews and confidential court documents seen by The Wall Street Journal portray the fugitives as part of an extensive web of young men who developed a deep hatred of the West after embracing radical Islam at underground mosques and clandestine meetings in Molenbeek, a heavily Muslim district in the heart of Brussels. They have since become central to Islamic State’s plans to strike the West, according to investigators, who suspect the Brussels network is behind the movement of battle-hardened operatives from Syria to Europe.

“We see many plots and several cells that we now know are part of the same network,” said Jean-Charles Brisard, president of the Center for the Analysis of Terrorism, a Paris think tank. “They’re already here. The problem is how to find them.”

The 22 men investigators are scrutinizing include those prosecuted in absentia during a major terror trial in Belgium last year, as well as several with links to a foiled plot to kill Belgian police last year. But people familiar with investigations say they believe the reach of the Brussels network extends beyond the group to others from the city who went to Syria and their sympathizers who stayed behind.

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