The woman was detained shortly after Michael Harpon, 45, knifed three male police officers and a female assistant at the office in central Paris on Thursday.
The 38-year-old was freed on Sunday, the France Info radio cited sources in the judiciary as saying. She has not been charged.
The probe into the murders that shocked France had been taken over by anti-terrorist prosecutors after it was revealed that Harpon, an IT expert, had shown signs of radicalization.
The man converted to Islam years ago changed his behaviour drastically in recent month. He cut off contacts with women and endorsed some of Islamist atrocities, including the 2015 attack on the Charlie Hebdo satirical magazine.