Initial investigations suggest gas was the cause of the blast, which destroyed the garage as well as two neighboring buildings and prompted around 50 firefighters to rush to the scene.
So far nine other people have been identified with minor injuries following the blast, while the 89-year-old is in a "serious condition".
The two missing people are the garage mechanic and his partner.
"At the moment we're worried that the number of victims will rise and that neighboring buildings and those further away could be weakened and may collapse," a spokesman for the firefighters said, AFP reported.
While gas supplies to the district have been cut, "we don't know yet whether it was the gas network or canisters" that caused the explosion, said Eric Destarac, a spokesman for gas distribution company Regaz.
Firefighters had recovered some gas bottles from the garage, he added.