Rescuers worked throughout the night to look for victims in the rubble of two dilapidated blocks of flats which collapsed suddenly on Monday morning not far from the centre of the Mediterranean port city.
Castaner said at the site that 120 police officers and 80 firefighters took part in the search-and-rescue operation, working through the night in the pile of beams and rubble.
But a 2015 government report said some 100,000 Marseille residents were living in housing that was risky to their health or security.
As the political row grew, the interior minister, Christophe Castaner, announced he had ordered a building-by-building audit in Marseilles before launching what he called an "ambitious programme" to ensure safer conditions.
"Nearly 6,000 properties have been identified as at risk" in the city, he said, representing some 44,000 lodgings in lower-class neighbourhoods.
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"People died for nothing even though we knew", he added.
People had been living in nine of the 10 apartments at number 65, while a shop occupied the ground floor.
Several others remain missing. Previously, the government said that under the debris may be from five to eight people. "It is the houses of poor people that collapse - and that is no coincidence", he was quoted as saying by local media.
"It could have been me", the 25-year-old philosophy student said, visibly shaken.
But a 2015 government report said about 100,000 Marseille residents were living in housing that was unsafe to their health or security.
Djaffar Nour, who was shopping down the street, said the collapse had happened in "a matter of seconds".