(CNN) Five terrorists -- three dead, two whose fates are unknown. So far, that's how many people that authorities say played a part in Tuesday's terror attacks in Brussels , Belgium, that killed 31 people and injured 330. Two were allegedly involved in bombing a train near the Maelbeek metro station. Of them, one has been identified as Khalid El Bakraoui, who died in that suicide attack. And on Thursday, a senior Belgian security source told CNN that authorities believe a second, unnamed person was also involved in that blast. That man's whereabouts -- and whether he's dead or alive -- is unknown. Surveillance footage shows this man holding a large bag at the station, according to Belgian public broadcaster RTBF. It's not clear if he was among the at least 20 killed in that blast, RTBF said. Three more people apparently took part in the Brussels Airport attack. At least two of them are dead -- Khalid's brother Ibrahim El Bakraoui and ISIS b...