Gouet-Jenselme deals primarily with insurance, real estate and divorce cases.
Officials said the package had been addressed to Olivier Brane, a laywer at the Gouet-Jenselme law firm who was severely injured in the blast.
Nicolas Sarkozy, France's president, once practised as a lawyer at another firm in the same building.
Jean-Claude Marin,Paris public prosecutor, said the explosion had no link to Sarkozy.
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, organisation that does research on the Nazi Holocaust.
Serge Klarsfeld, a French Nazi investigator, said he did not think the foundation had been the target.