An office secretary opened the wooden box package after it was delivered to the law office on Thursday, and two explosives inside went off, killing her and seriously injuring a lawyer.
Judicial police entered the home of the suspect's mother, where he was living, the police officials said on condition of anonymity because an investigation was ongoing.
The suspect, said to be a 45-year-old architect, had allegedly harassed a lawyer in the firm two years ago, the officials said.
The building also housed another law firm co-founded by French President Nicolas Sarkozy and a Holocaust research foundation, though officials said they did not appear to be targeted.
After the arrest Friday, authorities were ruling out any possibility of terrorism, and continued a search for a delivery woman who dropped off the package, the officials said.
Police investigators were questioning the suspect on Friday.
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