"We are really in shock. I don't understand how someone could kill a woman who has no money and who lives in a social housing complex," her son added.
The other suspect is a homeless man known to police, Le Parisien reported.
Jean-Yves Le Drian, the French foreign minister who happened to be in Jerusalem on Monday, said it was "plausible" that Knoll was murdered because of her religion.
France, he added, was engaged in a "fundamental and permanent" fight against anti-Semitism.
The chief rabbi of Paris, Haim Korsia, wrote on Twitter that he was "horrified" by the killing.
Investigators had not mentioned anti-Semitism on Sunday but added that they were "not excluding any hypothesis".
The Protection Service for the Jewish Community, SPCJ, a body which keeps close watch an anti-Semitic acts in France, had earlier said: "The investigation does not reveal any anti-Semitic elements, however, this path has not been ruled out to date and needs to be further explored."
Knoll had managed to evade the notorious 1942 roundup of more than 13,000 Jews in Paris by fleeing with her mother to Portugal thanks to her Brazilian passport.
All but 100 or so of those seized at the so-called Vel d'Hiv cycling track and then sent to the Nazi death camps survived.
Knoll had returned to Paris after the war and married a Holocaust survivor, who died in the early 2000s.
Anti-Semitic violence increased by 26 per cent last year in France, which has Europe's biggest Jewish community, and criminal damage to Jewish places of worship and burials increased by 22 per cent.
Last month, a judge confirmed that the April 2017 murder of Sarah Halimi, a 65-year-old Orthodox Jewish woman who was beaten and thrown out of her window, was motivated by anti-Semitism. Amid shouts of "Allah Akbar", her attacker beat Halimi before throwing her out of the window.
There had been an outcry when prosecutors had initially ruled out anti-Semitism.
In January an 8-year-old boy wearing the Jewish skullcap was beaten up by two teenagers in the northern Paris suburb of Sarcelles in what prosecutors said appeared to be attack motivated by the child's religion.
A record 7,900 French Jews emigrated to Israel in 2015 following the deadly jihadist shooting at a Parisian kosher supermarket two days after the attack on satirical magazine Charlie Hebdo. That exodus has since slowed.
The CRIF umbrella grouping of French Jewish organisations called for "the fullest transparency" over Knoll's murder, which it called a "barbaric crime".
At an annual dinner organised by the group last month, President Emmanuel Macron slammed anti-Semitism as "France's dishonour" and pledged never to let up the fight against the phenomenon.
Working definition: Anti-Semitism
The International Holocaust Remembrance Alliance hopes that providing a strict definition of anti-Semitism will help organisations to combat it. The definition is as follows:
"Anti-Semitism is a certain perception of Jews, which may be expressed as hatred toward Jews. Rhetorical and physical manifestations of anti-Semitism are directed toward Jewish or non-Jewish individuals and/or their property, toward Jewish community institutions and religious facilities."
The IHRA gives examples such as:
• Accusing Jews as a people of being responsible for real or imagined wrongdoing committed by a single Jewish person or group, or even for acts committed by non-Jews.
• Accusing Jewish citizens of being more loyal to Israel, or to the alleged priorities of Jews worldwide, than to the interests of their own nations.
• Denying the Jewish people their right to self-determination, e.g., by claiming that the existence of a State of Israel is a racist endeavor.
• Using the symbols and images associated with classic anti-Semitism (e.g., claims of Jews killing Jesus or blood libel) to characterize Israel or Israelis.
• Drawing comparisons of contemporary Israeli policy to that of the Nazis.
• Holding Jews collectively responsible for actions of the state of Israel.
Source: International Holocaust Remembrance Alliance
This article originally appeared on the Daily Telegraph.
