Belgium's capital woke Saturday to a security lockdown as the national crisis center raised its terrorism alert to its highest level and at least one suspect from the Paris attacks remains at large.

Authorities across Europe, the Mideast and in Washington are trying to determine how a network of primarily French and Belgian attackers with links to Islamic extremists in Syria plotted and carried out the deadliest violence in France in decades — and how many may still be on the run.

Belgium's national Crisis Center raised its terrorism alert for the Brussels region to Level 4, which indicates a "serious and immediate threat."

The Belgian capital was home to the suspected organizer of the Nov. 13 Paris attacks, Abdelhamid Abaaoud, and Belgium has filed charges of "participation in terrorist attacks and participation in the activities of a terrorist organization" against three suspects relating to the Paris attacks.

Heavily armed police and soldiers patrolled Saturday morning at key intersections of the Belgian capital, a city of more than 1 million that is home to the headquarters of the European Union, the NATO alliance and offices of many multinational corporations.

Residents were recommended to avoid gatherings, train stations, airports and commercial districts. Service was halted on the Brussels Metro, as well as on streetcar lines that run underground, and residents were urged to stay indoors.

In Turkey, authorities detained three suspected Islamic State militants, including a 26-year-old Belgian of Moroccan descent.

The state-run Anadolu Agency said Saturday that the two Syrians and the Belgian national — identified as Ahmet D. — were detained near the Turkish coastal city of Antalya. The private Dogan news agency identified the Belgian as Ahmet Dahmani and said he is suspected of having explored areas in Paris that were targeted in the attacks.

Shell-shocked Parisians honored the 130 victims Friday night with candles and dancing, marking exactly a week since attackers opened fire on sidewalk cafes and exploded suicide vests at the national stadium and an iconic rock concert venue.

Concerns about Europe's porous borders prompted interior and justice ministers meeting in Brussels on Friday to promise tightened border controls to make it easier to track the movements of jihadis with European passports traveling to and from warzones in Syria.

Prosecutors said Friday that they had determined through fingerprint checks that two of the seven attackers who died in the bloodshed Nov. 13 had entered Europe through Greece, an entry point for many of the hundreds of thousands of migrants seeking asylum in Europe.

The five other attackers who died had links to France and Belgium. One of the seven dead has not been identified, while a manhunt is underway for one suspect who escaped, Salah Abdeslam, 26. French police stopped Abdeslam the morning after Friday's attacks at the Belgian border but then let him go.

French officials said Saturday they could not ascertain for certain whether Abdeslam might be in France or Belgium. His brother Brahim, blew himself up in the Paris attacks.

The suspected ringleader, Abdelhamid Abaaoud, was killed in a raid early Wednesday on an apartment in the Paris suburb of Saint-Denis, along with Hasna Aitboulahcen, a 26-year-old woman who officials said had described herself as Abaaoud's cousin. Prosecutors said Friday that a third person was killed in the raid but did not release the identity.

They also said Aitboulahcen had not blown herself up with a suicide vest, as initially believed, which suggests the body parts collected after the raid belonged to the third, as-yet-unidentified, person.

Marking a week since the carnage, some Parisians lit candles and paid tribute to the victims with silent reflection. Others decided that enjoying themselves was the best way to defy the extremists. They sang and danced on Place de la Republique, in the heart of a trendy neighborhood where scores of people were killed, most of them in the attack on the Bataclan concert hall.

Demonstrations have been banned in the city since the attacks, but Parisians have been spontaneously gathering all week outside the restaurants, cafes and concert hall hit in the attacks to leave flowers, light candles or hold quiet vigils.

France's Senate on Friday voted to extend for three months a state of emergency, which expands police powers to carry out arrests and searches and allows authorities to forbid the movement of persons and vehicles at specific times and places. France's lower chamber has already approved the measure.

French President Francois Hollande is also going to Washington and Moscow next week to push for a stronger international coalition against IS.

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Angela Charlton, Frank Jordans, Karl Ritter and Maria Sanminiatelli in Paris and Lorne Cook in Brussels contributed to this report.