Americans in Belgium were ordered “to shelter in place” Saturday after local authorities warned that the threat of a terrorist attack in the country’s capital is serious and imminent.

The U.S. Embassy in Brussels also ordered American citizens to remain at home as the city was placed on lockdown in response to the terrorist threat. The national crisis center raised its terrorism alert Saturday to its highest level as Belgian police continued to search for a suspect in the Paris attacks that killed 130 people.

In a statement on its website Saturday the embassy informed Americans that “if you must go out, avoid large crowds.”

The warning also urged U.S. citizens to “exercise caution in public transportation systems, sporting events, residential areas, business offices, shopping malls and other tourist destinations.”

Belgian leaders raised the terrorism alert to Level 4, which indicates a “serious and immediate threat.” 

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Prime Minister Charles Michel said the decision to raise the alert level was taken "based on quite precise information about the risk of an attack like the one that happened in Paris...where several individuals with arms and explosives launch actions, perhaps even in several places at the same time."

Authorities across Europe, the Middle East and Washington are trying to determine how a network of French and Belgian nationalists with links to Islamic extremists in Syria plotted and carried out the deadliest violence in France since World War II – and how many may still be on the loose.

Brussels was home to the suspected organizer of the Nov. 13 terror 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 key intersections of the Belgian capital Saturday morning, a city of more than 1 million that is home to the headquarters of the European Union, the NATO alliance and officers 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.

A new potential link emerged Saturday in Turkey, where authorities detained a suspected ISIS militant from Belgium who was believed in contact with the Paris attackers.

Ahmet Dahmani, 26, a Belgian of Moroccan descent, was detained with two other suspected ISIS militants. A senior Turkish government official told the Associated Press Dahmani was believed to have been in contact with the Paris attackers but the official did not say when. Dahmani arrived in Turkey Nov. 14 from Amsterdam and the three were preparing to cross into Syria, the official said.

Parisians across the French capital honored the 130 victims Friday night with candles and dancing, marking one week since attackers opened fire on sidewalk cafes and exploded suicide vests at the national stadium and an iconic rock venue.

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 it was initially believed, which suggests the body parts collected in the rubble after Wednesday’s anti-terror raid belonged to a third person who has yet to be identified.

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.

The Associated Press contributed to this report.