Two of the Paris suicide bombers, Brahim Abdeslam and Bilal Hadfi, had been living in Belgium. Fugitive suspected militant Salah Abdeslam, Brahim's 26 year-old brother, slipped back home to Brussels from Paris shortly after the attacks.
Asked whether Brussels' maximum threat level since Saturday related to Salah Abdeslam alone, Interior Minister Jan Jambon told broadcaster VRT "unfortunately not".
"It is a threat that goes beyond just that one person," he said. "We're looking at more things, that's why we've put in place such a concentration of resources."
Bernard Clerfayt, the mayor of the Brussels district of Schaerbeek, was quoted by broadcaster RTBF as saying there were "two terrorists" in the Brussels area ready to carry out violence.
Mohamed Abdeslam, the brother of Brahim and Salah, urged Salah in an interview on RTBF television to give himself up, adding that he believed Salah was still alive because he had had a last-minute change of heart while in Paris.
Belgium's crisis centre, a state body that advises the government on security, said on Sunday the alert status for Brussels remained at its highest level of four, meaning a "serious and imminent" threat of an attack.
Intelligence, police and judicial officials would review the alert status during the course of the day. The national security council, including top ministers, was expected to convene on Sunday afternoon to determine what measures to take or retain.
Justice Minister Koen Geens told VRT the metro system was likely to resume on Monday.
"We will guard the metro stations ... We are not going to paralyse Brussels economically, nor the country. We are not led by panic and fear, but we have needed time to reorganise everything," Geens said.
Prime Minister Charles Michel has advised the public to be alert rather than panic-stricken, but also said Brussels risked Paris-style coordinated attacks.
Belgium has urged the public to avoid crowds in the capital, and also closed museums, cinemas and shopping centres. Clubs and venues have cancelled events.