More than 25 people are reported dead after explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe.

European security officials have been bracing for a major attack for weeks, and warned that the Islamic State group was actively preparing to strike.

The arrest Friday of a key suspect in the November attacks in Paris heightened those fears, as investigators said many more people were involved than originally thought, and that some are still on the loose.

Here is how the attacks have been linked: