BRUSSELS - AbdelhamidAbaaoud, suspected planner of the Paris attacks, mocked European frontier controls and boasted of the ease with which he could move between Syria to his Belgian homeland and the rest of Europe.

As police piece together how militants mounted Friday's assaults, evidence that some involved had fought in Syria and were on wanted lists, yet slipped back to kill 129 people, will raise a host of questions on how Europe tracks local Islamists and controls the borders it has opened to half a million Syrian refugees.

Abaaoud, a 28-year-old of Moroccan origin, was the best known of more than 350 Belgians to fight in Syria -- proportionately the biggest contingent from Europe -- and in particular of dozens of young men from the poor Molenbeek district of Brussels to take up arms for Islamic State.

His notoriety was based on videos on social media and, for the past year, his boasts of mounting attacks in Europe.

In a slick online English-language magazine produced by Islamic State in February, Abaaoud, now known as Abu Omar al-Baljiki -- the Belgian -- said he had been in his homeland the previous month planning attacks with two others who had fought in Syria and who died in a Belgian police raid in January.

He had, he said, returned to Syria, despite his face having been well known to police thanks to a much-posted video of him driving a pick-up truck towing the bloodied bodies of prisoners. (Reuters)