A suicide bomb belt was found on a Paris street following attacks that killed 130 people on November 13.
It is said to resemble belts used by the attackers and was found in a suburb which a suspect is thought to have passed through after the attacks, French police say.
Meanwhile, the US Department of State has issued a worldwide travel alert in response to Paris attacks.
The Belgian capital Brussels remains on high alert. Schools and the metro will stay closed on November 24.
They are due to reopen on November 25 but the highest alert level will continue for at least another week.
Authorities fear attacks like those in Paris may be carried out in Brussels, where at least one Paris attacker lived. Belgian PM Charles Michel warned that the threat remained “imminent”.
French President Francois Hollande is due to meet President Barack Obama in Washington on November 24 as he continues a busy week of international diplomacy during which he will meet all other permanent members of the UN Security Council.
The suspected explosives belt was found in a dustbin on November 23 by street cleaners in the Montrouge district, police say.
Police sources told news agencies it resembled those used in the November 13 attacks. According to AFP news agency, the device lacks a detonator.
It is one of two key pieces of evidence discovered by French police and publicly linked to Salah Abdeslam, one of the main suspects.
Salah Abdeslam’s brother, Brahim, blew himself up during the attacks.
A mobile phone was previously found in an abandoned car he rented. Phone data suggest that on the night of the attacks, Salah Abdeslam was in the area where the belt was later found.
It may be that he planned to detonate the bomb belt but abandoned the plan – either because the belt was malfunctioning or, as his brother Mohamed has suggested, because he had a last-minute change of heart.
A massive manhunt for Salah Abdeslam is continuing in both France and Belgium.