Shafaq News/ The Belgian federal police said Wednesday it remained “vigilant” after it received an email Tuesday threatening a terror attack targeting the Brussels underground metro system.

“At the moment, we remain very vigilant, as is necessary,” said a spokesperson for the Belgian federal police, in charge of security in metro stations.

The police swept the metro during the night but “found nothing suspicious,” the spokesperson added.

According to a spokesperson for the Brussels civil prosecutor — which has taken over the investigation — the risk of a “real attack” is considered “unlikely.”

A Belgian National Crisis Center spokesperson — charged with organizing the security response in case of a terror attack — said the threat was being evaluated but confirmed that an attack was unlikely.

The American embassy in Brussels picked up the threat and issued a warning to U.S. citizens, recommending they “use caution when traveling in and around Brussels.”

Brussels suffered a major terrorist attack at the city’s international airport and the Maelbeek metro station in March 2016, killing 32 people.