By Adam Nossiter

New York Times News Service

Travel alert issued — The State Department alerted U.S. citizens Monday to possible travel risks associated with an increased threat of terrorism. The department said information suggested terrorist groups including the Islamic State, al-Qaida and Boko Haram are continuing to plan attacks in several regions. The alert will remain in force for three months, until Feb. 24, it said. Americans were called on to exercise vigilance in public places and when using transportation networks. Particular caution was urged during the holiday season and at holiday events. In addition, it said, Americans should monitor media and local information sources and adjust their plans accordingly. Another recommendation was to prepare for “unexpected disruptions” and added security screening.

Suicide vest found in Paris — A street cleaner in a Paris suburb Monday found an explosive vest similar to those used in the Paris attacks near the place where a suspect’s mobile phone had been found, raising the possibility he aborted his mission, either ditching a malfunctioning vest — or fleeing in fear. Police said the 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. The device was found Monday 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.

Jittery silence in Brussels — On what would normally have been a busy Monday, this city in the heart of Europe remained shut down for a third straight day. Everything was closed — schools, museums, government offices, public transportations, markets, shops. The threat that terrorists aligned with the Islamic State would attack remained both real and unspecified. Authorities said it was real enough to continue to have soldiers in the streets, conduct raids, arrest people of suspicion and tell everyone to avoid crowds and unnecessary travel. What they did not say is exactly what warranted such extreme measures.

— From wire reports

PARIS — All over France, from Toulouse in the south to Paris and beyond, the police have been breaking down doors, conducting searches without warrants, aggressively questioning residents, hauling suspects to police stations and putting others under house arrest.

The extraordinary steps are perfectly legal under the state of emergency decreed by the government after the attacks Nov. 13 in Paris that left 130 dead — a rare kind of mobilization that will continue. The French Parliament last week voted to extend the emergency for another three months, which means more warrantless searches, more interrogations, more people placed under house arrest.

There have been 1,072 police searches, 139 police interrogations, and 117 people have been placed in custody, the Interior Ministry said Monday. Those included a weekend raid on a restaurant selling halal burgers and Tex-Mex food in the Paris suburbs, where officers found nothing suspicious, after breaking down the doors.

Many of those being swept up are among the hundreds of French who have already been flagged as potential security threats in the notorious S-files of the security services.

The police are now free to pick up and interrogate suspects virtually at will.

An indication of the lingering shock of the attacks — and the fear coursing through French society — is that few, publicly at least, are protesting these exceptional measures. But critics of the broad net now being cast by the security services say the results are meager given the looming threat to civil liberties.

Concern is rising, particularly in Muslim communities being singled out, that France now runs the risk of tipping steeply in favor of security at the expense of individual freedoms and of instigating tension with a Muslim population — the largest in Europe — that has long felt aggrieved and second class.

“These measures are going to place a spider’s web over all of France,” said Danièle Lochak, an emeritus professor of law at the University of Paris. “But in a discriminatory manner, because it will concern Muslims. It’s out of control. What are they going to do with all these people who are under house arrest?” The answer, so far, is not clear.

“You’ve got the feeling that the police can do whatever they want now,” she said in a telephone interview Monday. “We are going to forgo all of the protective rules.

“This is what bothers me, this headlong rush forward by the government,” she added. “You’ve got the impression that nothing else matters — it is all just collateral damage.”

Even critics of the aggressive policing acknowledge the government must be seen doing something to protect the shaky public.

At the same time, though, the critics worry the drift toward a state where the police have the upper hand has discomfiting echoes in modern French history — the collaboration of the Vichy regime in World War II, for example — and they say authorities must be careful not to give in to panic.

“Police searches and house arrests can now be ordered by the Interior Ministry and the prefects” — local officials under the control of Paris — “without judicial warrant,” noted Bénédicte Jeannerod of Human Rights Watch.

“These extrajudicial searches have shot up since the attacks, they are being carried out in haste, and under pressure from public opinion and the political class,” she said. That context, Jeannerod added, can only encourage human rights abuses and mistakes.

Long faces in the capital, more soldiers patrolling landmarks like the Louvre Museum and empty streets speak to the preoccupation with security. Politicians all over the country say their constituents are talking of little else but the slaughter a little less than two weeks ago, according to the French media.

Amid the continued jitters, the French police said they found a suicide belt Monday afternoon in a garbage bin in Montrouge, in Paris’ 18th Arrondissement, said a former French intelligence official close to the investigations.

The police say it may have belonged to Salah Abdeslam, who had a role in the Paris attacks, because a cellphone of his had been matched to the location where the suicide belt was found. The former intelligence official also said the Islamic State had claimed that the 18th Arrondissement was a target, but that an attack never materialized.

“We’re in a period of total tension,” said Xavier Nogueras, a Paris lawyer who represents a handful of the 180 or so people placed under house arrest.

Nogueras’ clients are all Muslim, and are in the S-files that register those considered possible threats to the state. Participants in all the major terrorist attacks in France this year were in the S-files, too, including those in the attacks Nov. 13.

But Nogueras says his clients, under the house-arrest procedure, are shouldering an intolerable burden they do not deserve. Simple attendance at a mosque under surveillance can land you in the S-files, he noted. The result has been a catastrophe for his clients, he said.

Under the house-arrest rules, they must report to the local police station up to four times a day. “That is totally excessive,” Nogueras said. “These measures threaten individual liberties. For most of them,” he said, “who have a normal work life, they can’t even work any longer.

“They are taking it extremely badly. Because mostly they have nothing to reproach themselves for. They are living a Western lifestyle.”

The Interior Ministry defended the exceptional measures in a news release Friday. “These operations are going to continue,” the ministry’s statement said, noting “the government’s total determination to fight without mercy against terrorism, and every threat to public order.”

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