A 60-year-old secretary at the firm, Jacqueline Belbouai, was killed when parcel addressed to the firm exploded as she was opening it. A 58-year-old lawyer was also seriously injured by the explosion, which occurred at 1: 50 p.m. GMT.
The building where the explosion took place also houses French President Nicolas Sarkozy's former law office and a Holocaust memorial group.
However, a presidential spokesman said there was no evidence that Sarkozy was the target.
The suspect, who is believed to be in his mid-forties, is an architect who had earlier reportedly harassed a female lawyer at the firm, police said. The search for a "blonde woman" who is said to have delivered the parcel is under way.
Sarkozy, who had just returned from a three-day visit to Algeria, condemned the attack, saying that it was "a truly barbaric act". French Interior Minister Michele Alliot-Marie cut short her visit to Brussels, and returned to Paris to oversee police and rescue work at the site of the blast.
