Law firm 'mystified'
Police said another suspicious package was defused in the same building.
Officials said the package had been addressed to Olivier Brane, a lawyer at the Gouet-Jenselme law firm who was severely injured by the blast.
Jean-Claude Marin, a Paris public prosecutor, said the law firm, which deals primarily with insurance, real estate and divorce cases, was "mystified by this incident".
Sarkozy, a former lawyer, retains a 30 per cent stake in the Arnaud Claude and Associates firm which shares the building, but Marin said the explosion had no link to the president.
France's interior minister left Brussels, where she was to attend an EU meeting, to visit the scene of the explosion.
In a statement, Michele Alliot-Marie said she "condemns with the greatest severity this cowardly and hateful act".
Bertrand Delanoe, mayor of Paris, said the building also houses the Shoah Foundation, an organisation that does research on the Nazi Holocaust.
But Serge Klarsfeld, a French Nazi investigator, said he did not think the foundation had been the target.
In Paris, Al Jazeera's Tim Friend said the police were looking for a person thought to be a woman seen sending the parcel to the firm.