French police are examining what seems to be a suicide bomb belt dumped on a Paris street, 10 days soon after the attacks that killed 130 people.
It is said to resemble belts employed by the attackers and was found in a suburb which a suspect is thought to have passed through right after the attacks.
The US has issued a worldwide travel alert in response to the attacks.
The Belgian capital Brussels remains on high alert, and schools and the metro will stay closed on Tuesday.
They are due to reopen on Wednesday but the highest alert level will continue for at least an additional week.
Authorities worry Paris-style attacks may possibly be carried out in the city, exactly where at least one particular Paris attacker lived, with Prime Minister Charles Michel warning the threat remained “imminent”.
The suspected explosives belt was identified in a dustbin on Monday by street cleaners in the Montrouge district, police say.
Police sources told news agencies it resembled those utilized in the 13 November attacks. According to AFP news agency, the device lacks a detonator.
Investigators say mobile phone data recommend that Salah Abdeslam – whose brother Brahim blew himself up for the duration of the attacks – was in the location on 13 November.
1 theory is that Salah Abdeslam had planned to detonate the bomb belt but abandoned the program.
“Possibly he had a technical dilemma with his explosive belt,” a police supply told Reuters news agency.
A massive manhunt for Salah Abdeslam is continuing in both France and Belgium.
Brussels on Monday saw a third day of unprecedented restrictions, with troops and armed police patrolling the streets.
Announcing that the state of alert would stay at level four, Mr Michel stressed “we need to all progressively get back to a standard life”.
The rest of Belgium remains on alert level three, which means an attack is noticed as possible and credible.
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Belgian police have charged a fourth suspect with terrorism offences connected to the Paris attacks, the federal prosecutor mentioned.
The unnamed man was one of 21 folks detained in raids on Sunday and Monday. Seventeen have been released with out charge.
Mohammed Amri, 27, and Hamza Attou, 20, have already been charged with aiding Paris attacks suspect Salah Abdeslam, who remains at large. A third, unnamed suspect has also been charged.
Also on Monday, France carried out its initial strikes against IS from its Charles de Gaulle aircraft carrier, newly deployed in the eastern Mediterranean.
French jets bombed IS targets in Iraq and Syria, like Raqqa, IS’s important Syrian stronghold, the defence ministry said.
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President Francois Hollande has vowed to intensify strikes against IS in Syria and Iraq in the wake of the attacks.