The discovery of the vest came as Belgium's prime minister cited a "serious and imminent" threat justifying keeping the highest alert level operational for at least another week. The security measures, already in place for three days, have severely disrupted normal life in the capital.
In France, police said an explosive vest - without a detonator - was found by a street cleaner in a pile of rubble in Chatillon-Montrouge, on the southern edge of Paris and a considerable distance from the sites of the attacks on the Right Bank of the Seine to the north. A police official later said the vest contained bolts and the same type of explosives - TATP - as those used in the Nov. 13 Paris attacks that claimed 130 lives and left hundreds wounded.
The device was found in the same area where a cellphone belonging to fugitive suspect Salah Abdeslam was located on the day of the Paris attacks but the vest has not been formally linked to him, said two police officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.
Belgium-based terrorism expert Claude Moniquet, who has been in contact with both Belgian and French investigators since the attacks, laid out two possibilities: that Abdeslam became afraid of carrying out a suicide mission or, more likely he says, that he simply ditched a defective explosive vest.
Nervousness could have played a role in concocting a defective vest, but he said he doubted fear played a role for among Islamic State followers, "it is rare not to go to the end."
Moniquet said this was only theory since he had not yet spoken to investigators about the explosive vest find.
A manhunt is underway for Abdeslam, whose brother Brahim was among attackers who blew themselves up. He crossed the border into Belgium after the attacks, with French police stopping and interviewing him, before letting him go.
Belgian Prime Minister Charles Michel said Brussels, which houses the headquarters of the European Union and NATO, faced a "serious and imminent" threat that requires keeping the city at the highest alert level, while the rest of the country would stay at the second-highest level. Belgium's crisis center said the alert level would only change if a significant breakthrough warranted it.
The increased security measures in the wake of the massacre in Paris have virtually shut down the Belgian capital, with the subway system, many shops and schools remaining shut on Monday. Michel said that despite the continued high-alert level, schools would reopen Wednesday, with parts of the subway system beginning to operate. He did not say when the system would be completely online again.
"We are very alert and call for caution," Michel said. "The potential targets remain the same: shopping centers and shopping streets and public transport."
"We want to return to a normal way of life as quickly as possible," he added.
Belgian authorities have not announced any details of their investigation into potential attacks nor have they released information about four suspects who have been arrested and charged with terrorism-related offenses. These include one suspect who was arrested as part of a sweep that saw 21 people detained since Sunday night. Fifteen of those detainees have since been released.
Frank Foley, a terrorism expert and lecturer in war studies at King's College London, said it was difficult to know if the Belgian operations were justified because authorities have provided few details. The measures could even be counterproductive if they last too long, he said.
"If these dramatic measures continue in Brussels, we will be doing the terrorists' job for them," Foley said. "The government may be unintentionally contributing to the atmosphere of fear."
Henry Willis, director of RAND Homeland Security and Defense Center, likened the clampdown to the reaction of U.S. authorities after the Boston Marathon bombing.
"They did shut down the city for a couple of days and when they lifted those restrictions, that's when they caught the terrorist," he said.
Several of the Paris attackers had lived in Brussels, including Abdelhamid Abaaoud, the man who authorities say orchestrated the plot. He was killed Wednesday in a standoff with French police.
French authorities issued a new appeal for help in identifying one of the three attackers who was killed in the assault near the national stadium. They posted a photo of the man on Twitter Sunday asking the public for information.
Greek police confirmed the man posed as an asylum seeker before the carnage. Public Order Minister Nikos Toskas said the man traveled to the island of Leros on Oct. 3, but he gave no further details.
Two senior Greek law enforcement officials told The Associated Press that the man traveled with another attacker, identified as Ahmad Al Mohammad. Both men were rescued by the Greek coast guard while traveling from Turkey on a boat carrying nearly 200 migrants and refugees that sank before reaching Greece. The officials requested anonymity because they weren't authorized to speak to the media.
Last week, France extended for three months a state of emergency that allows police raids, searches and house arrest without permission from a judge. On Saturday, it also extended a ban on demonstrations and other gatherings through Nov. 30, when a U.N. climate conference to be attended by more than 100 heads of state is scheduled to start.
The West, reeling from attacks in the heart of Europe, was also using military might to go after the Islamic State group.
Earlier Monday, British Prime Minister David Cameron said during a visit to Paris that he would seek parliamentary approval for the U.K. to join the airstrikes being carried out by the U.S., France, Russia and other nations against the Islamic State extremists in Syria. Britain has been carrying out airstrikes in Iraq, and Cameron has long wanted an expanded mandate to extend the air campaign to Syria.
Cameron and French President Francois Hollande paid a visit to the Bataclan concert hall, which saw the worst of the carnage. Seeking a unified strategy on Syria, Hollande meets Tuesday with President Barack Obama and with German Chancellor Angela Merkel on Wednesday, before traveling to Moscow on Thursday to meet with Russian President Vladimir Putin.
France's Defense Ministry said it launched its first airstrikes from the Charles de Gaulle aircraft carrier, bombing IS targets in the Iraqi cities of Ramadi and Mosul in a seven-hour operation. The ministry said four Rafale fighter jets were sent from the carrier Monday afternoon. France has already carried out strikes against IS targets in Syria.
