Belgian special forces arrested four people Saturday in Brussels, the Independent reported, citing local media. The country remained on the highest terrorism alert possible, Level 4. That alert had shut down Brussels' subway and was based on a "serious and immediate threat," the Interior Ministry said.

It was unclear to what extent the detentions on the historic Place du Grand Sablon at about 3 p.m. local time (9 a.m. EST) were linked to the investigation into a string of deadly terrorist attacks in and around Paris Nov. 13. The Islamic State group, formerly known as either ISIL or ISIS, claimed responsibility for them.

Since those attacks, Belgian police have carried out raids across the country. They arrested one person and found weapons and ammunition in the Brussels suburb of Molenbeek Friday. Authorities have said that suburb was home to two of the Paris attackers: Ibrahim Abdeslam, who died in the attacks, and his brother Salah Abdeslam, who remains on the run.

The fact Brussels issued its pointed terrorism alert indicated officials there "have something specific and credible at the intelligence front pointing them in the direction that there may be a terrorist plot in the works," CNN terrorism analyst Paul Cruickshank said. "It also suggests they don't have a handle on it, that they don't know where these plotters are or where they're coming from."

It was unclear who was arrested in Brussels Saturday, Belgian media reported. Salah Abdeslam was reported Friday to be hiding in the vicinity of the city. His brother, Mohamed Abdeslam, a Brussels resident, has urged him to surrender to police. Meanwhile, an international warrant has been issued for Salah Abdeslam's arrest, ABC News reported Friday.