French investigators are treating the deaths of at least six people who were killed when a building collapsed in Marseille as an “involuntary homicide” caused by a gas explosion, a prosecutor said.

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

Four of the victims have been identified as a 74-year-old couple and two women, aged 88 and 65, who were neighbours.

Rescuers continued searched on Tuesday for two people who remained unaccounted for following the blast 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, Ms Laurens said during a 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 centre of Marseille collapsed, killing eight people. Those buildings were poorly maintained, which was not the case with the one that collapsed on Sunday, the interior minister said.