French investigators were working to determine whether the gunman who attacked people in Strasbourg Christmas market acted alone or with accomplices, Paris prosecutor Remy Heitz said on Friday.
"The investigation will continue to identify possible accomplices who may had helped him or encouraged him to commit his crime," Heitz told reporters.
"We are now reconstructing the events of last 48 hours to see if he had benefited from any support," he added.
On Tuesday evening, a man armed with a firearm and a knife, opened fire several times and stabbed people in Place Kleber and the Grand'Rue, one of the French city's main shopping streets where Christmas market is held every year.
Three people were killed and one victim was brain-dead. A total of 12 passers-by were wounded, one still fighting for his life, according to Paris prosecutor.
The shooter, identified as Cherif Chekatt, has been killed, on Thursday evening, 48 hours after the attack, in the Neudorf-Meinau district of the city, two kilometers from where he targeted people near the Christmas market.
"At this point, seven people have been put in police custody. Four members of his family have been detained since Tuesday and three members of his close circle," Heitz said.
The 29-year-old man had been on police watchlist as he developed radical religious tendencies while he was in jail. He had been convicted several times in France, Germany and Switzerland for common law offenses, including violence and robberies.
A statement released Thursday on the Islamic State (IS) website claimed that Chekatt is one of its soldiers who "carried out the operation in response to calls for targeting citizens of coalition countries."
Attending the reopening of the Christmas market in Strasbourg on Friday, Interior Minister Christophe Castaner described the terrorist group's claim as "totally opportunistic".
Prior to that, Castaner told Europe 1 radio "nothing indicates that (Chekatt) was part of a network. There is nothing to suggest that he was being protected by such, but the investigation is not yet over."
Following the assault on Tuesday, France has raised its security threat level to the highest and poured additional 1,800 security forces across its cities to guarantee security in Christmas markets and for year-end holidays.