Several metro stations in Brussels were closed on Wednesday after a shooting at the central Clemenceau station, the Belgian prosecutor's office said.
No injuries were reported, it said.
The suspects fled into the metro tunnels afterwards, a source close to the investigation said, adding that police were looking for two suspects.
Sources close to the investigation told local media the incident may have been drug-related.
Images on VRT's website showed two people walking into the station and opening fire. The images could not immediately be verified by Reuters.
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