PARIS (AP) — Two bodies were found overnight in the rubble following an explosion that collapsed a building in the southern French city of Marseille, as rescuers continued searching for at least six people who are unaccounted-for, authorities said Monday.

Firefighters noted “the particular difficulties of intervention” and said in a statement the judicial authority will proceed to identify the victims.

Marseille mayor Benoit Payan tweeted Monday that “the pain and sorrow are great." He expressed his thoughts for the families of the victims and "those who are suffering."

"Rescue and search operations are continuing, without respite,” he said.

Payan told French media that more than 100 firefighters on site were searching for at least six people believed to have been trapped in the five-story residential building. “There is still hope” to find survivors, he said.

The burning debris was too hot for dogs in the firefighters’ canine team to work until Sunday afternoon, and smoke still bothered them, Laurens said.

An investigation has been opened for involuntary injury, at least initially sidestepping possible criminal intentions. A gas explosion was among the avenues of investigation, the prosecutor added.

The collapse occurred shortly before 1 a.m. on Sunday, in an old quarter in the center of Marseille, France’s second-largest city, less than a kilometer (a half-mile) from its iconic old port. About 200 people have been evacuated from their homes in the area.

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