PARIS — A blast from an explosive device has seriously injured three people at a residential building in Monaco, and the attacker fled to France, local authorities said. French and Ukrainian media reported that a Ukrainian magnate and his family were those injured.

The unusual attack Monday night stunned the elite principality on the Mediterranean Coast. Monaco’s leader Prince Albert II called it “an odious act” and said all the country’s services were mobilized to ensure security.

French and Monaco authorities are searching for the attacker, whose motive is under investigation, Monaco’s most senior government official, Minister of State Christophe Mirmand, told reporters.

The explosion occurred around 9 p.m. at the entryway of a residence near the French border, injuring two adults and a child who were taken to hospitals in France, he said.

The suspect crossed the border into France on foot, and was identified via video surveillance in Monaco and the neighboring French town of Beausoleil, Mirmand said.

A French national police official said French police are searching for the suspect and supporting the investigation, but would not elaborate.

French broadcaster BFM and Ukrainian news site Ukrainska Pravda identified one of the injured as Ukrainian construction tycoon Vadym Iermolaiev. Ukrainska Pravda said he was targeted by Ukrainian sanctions in 2023 for ties to Russia.