French investigators are treating the deaths of at least six people who were killed when a building collapsed in the city of Marseille as a possible "involuntary homicide" case, a prosecutor said Tuesday.

Prosecutor Dominique Laurens said an investigation was opened on that basis after the first body was found in the building, which collapsed in a fiery explosion early Sunday.

Four of the six victims have been formally identified, Laurens said: a 74-year-old couple and two women, ages 88 and 65, who were neighbors.

DEATH TOLL RISES TO 5 IN FRENCH BUILDING EXPLOSION

Firefighter

A firefighter walks near the scene where a building collapsed, in Marseille, southern France, on April 10, 2023. (AP Photo/Bishr El Touni)

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Rescuers continuing searching Tuesday for two people who remained unaccounted for following the emergency in France's second-largest city.

Investigators "are now working on the hypothesis of a gas explosion" as the cause of the building's collapse, Laurens said during a Tuesday news conference. A gas meter was found in the rubble that may help determine whether there was atypical consumption in the 24 hours prior to the explosion.

In 2018, two buildings in the center of Marseille collapsed, killing eight people. Those buildings were poorly maintained, which was not the case with the one that collapsed Sunday, the interior minister said.