BBC tracks down smuggler behind Channel crossing which killed Sara, 7

As he ambled, nonchalantly, across a sunlit public square, the smuggler appeared to have no idea he was being followed.

He was a short, stocky, 39-year-old in a pale green shell suit and baseball cap - an unremarkable figure taking an afternoon stroll from a tented migrant reception centre to a nearby tram station.

Our team broke into a run.

“We know who you are,” I said, as we caught up with him halfway across the square in Luxembourg’s capital city. “You’re a smuggler.”

It was a confrontation that marked the culmination of a BBC investigation that had begun 51 days earlier - hours after five people, including a seven-year-old girl named Sara, had died in the sea off northern France. She had suffocated beneath a crush of bodies inside an inflatable boat.

That investigation had taken us from the informal migrant camps around Calais and Boulogne, to a French police unit in Lille, to a market town in Essex, to the Belgian port of Antwerp, Berlin, and finally to Luxembourg and a three-day stakeout at the gates of the country’s migrant reception centre.