LONDON (AP) — London police are questioning four terrorism suspects believed to have been plotting an attack inside Britain and checking a possible link to the Islamic State group.

Officials said Wednesday that a medical student is among the group of four suspects, which includes one man who had traveled to Syria. The men, aged 20 and 21, were arrested Tuesday morning in London. They remain in custody and haven’t been charged.

Police commissioner Bernard Hogan-Howe told BBC radio the arrests involve a “quite serious case.” He said police are taking a more aggressive stance because of the increased threat linked to the fighting in Iraq and Syria.

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