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The suspect, a 32-year-old man, was held for questioning on the request of anti-terrorist judges in Paris, following the arrest on Tuesday of three men and two women from the same family.
All six were still being questioned late Wednesday, the source said.
Cherif Chekatt, a small-time criminal, shot dead five people and wounded 11 during the December 11 attack on the open-air Christmas market.
This week's arrests are part of investigations linked to the weapons held by Chekatt.
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Chekatt, 29, who was on a special watchlist as a suspected radical Islamist, was shot dead by police after a two-day manhunt in the city on the German border.
The Islamic State group claimed Chekatt was one of their "fighters", but this was dismissed by French Interior Minister Christophe Castaner.
Judicial sources said that Chekatt had pledged allegiance to the Islamic State group in a video found on a USB key.