Paris rescuers are struggling to reach a person feared trapped in a Left Bank building that partially collapsed earlier this week in a violent explosion, the city prosecutor said, citing a complex and risky rubble-clearing operation.

Four people remain in hospital in a critical condition following Wednesday’s blast, and at least 54 others suffered less serious injuries or psychological shock, prosecutor Laure Beccuau said.

One person has been declared missing but the search “has been complicated by the ongoing risk that the neighbouring building could collapse, and the difficulty of clearing large blocks of stone”, she said.

Buildings in the area were evacuated after the explosion, which ignited a fire that took hours to extinguish and sent smoke billowing over landmarks in the French capital, leaving witnesses shaken.

Investigators are studying whether a gas leak could have sparked the blast, among other possible causes. Ms Beccuau said witnesses had described the smell of gas.

The investigation will seek to determine whether the blast was the result of violation of safety rules, the prosecutor said.

Scientific experts can not search the site until it is fully secured.

The explosion near the historic Val de Grace military hospital in Paris’ 5th district crumpled the facade of a building that housed a private academy of design and arts.