(Reuters) -Belgian police have arrested an attacker who knifed three people in a Brussels metro station on Monday, leaving one with critical injuries, officials said. While the motives of the attacker, a 30-year-old man, remain unclear, police have ruled out a terrorist attack, the prosecutor’s office said, confirming the arrest. A source close to the investigation told Reuters the attacker was previously known by the police. “Cooperation with the various federal police services and (the local police) ensured that the perpetrator could be arrested quickly”, Brussels’ mayor Philippe Close said …