A Dutch couple of Turkish origin were arrested at Brussels Airport on Sunday after officials expressed suspicion of terrorist activities, CNN reported.
Three other people have also been arrested on terrorism suspicions in Belgium, Turkey and the Netherlands, according to CNN.
The five people arrested frequently traveled between Belgium and the Netherlands, CNN reported. All five remain in custody.
Belgian, Dutch and Turkish police made the arrests after a month of investigating the five people, CNN reported. The investigation consisted of surveillance of an alleged terrorist cell linked with Syrian jihadist groups.
"Their links to groups in Syria have been complex to untangle, but point towards ISIS," an unidentified official told CNN.
The couple was arrested Aug. 6 on their way back from a three-day trip to Istanbul, according to CNN. The third suspect was arrested in Belgium; the fourth suspect was arrested in the Netherlands, and a fifth was arrested in Turkey.
Police received information the alleged cell had gotten handguns which were recovered in a raid on one of the suspects' Brussels residence on Aug. 6, CNN reported. A cell of terrorists usually consists of three to five members and is used to insure operational security.
In the Netherlands, electronic media was seized during a raid in The Hague, including terrorist propaganda materials, the official said, according to CNN.
"It was obviously worrying when we discovered they'd obtained weapons," the official told CNN. "We were concerned they might be planning an attack here [in Belgium], but we still don't know what if, anything, they were planning."
Some media outlets have reported the group was planning an attack on the European Commission in Brussels, but no official confirmation has been given on those rumors, CNN reported.