Brussels - State-run Belgian broadcaster RTBF said on Thursday four suspected extremists detained by police were collecting money to buy arms and explosives and developing plans to strike at a crowded target like Antwerp's main train station.Belgian prosecutors have said all four suspects have been charged with participating in the activities of a terrorist group, but declined to give additional details.RTBF, citing "official sources", said the suspects were exchanging encrypted messages with other Belgians in Syria including Hicham Chaib, a leader of the Islamic State extremist group in its Syrian stronghold of Raqqa.The broadcaster says it was those communications that worried Belgian authorities, who detained the suspects on Wednesday along with a number of minors following police searches in the Antwerp area.RTBF said one suspect and one minor were still being held on Thursday.In a statement, the Belgian Federal Prosecutor's Office said the suspects don't appear to have links to the suicide bombers who struck the Brussels airport and subway March 22, killing 32 victims. Islamic State claimed responsibility for the carnage, as well as for the November 13 attacks that killed 130 people in Paris.Belgium, home to a large minority population of North African origin, has been one of Europe's most fertile recruiting grounds for ISIS. Prosecutors said some of those detained on Wednesday planned to go to Syria or Libya and join the extremist group. All of the suspects are believed to have wanted to recruit people to go to those conflict zones, and initial indications are they were plotting attacks in Belgium as well, prosecutors said.RTBF said the suspects' goal was to "strike at a crowd" and that the train station in the port city of Antwerp, one of Belgium's busiest, was a potential target. The broadcaster said the investigation that led to Wednesday's searches and detentions started in November after the Paris attacks.