BIDEN-HARRIS >> WE ARE GLAD TO HAVE YOU WAL US THROUGH THAT WITH A BUSY TW MONTHS AHEAD >>> POLICE SAY A SHOOTING THAT KILLED FOUR PEOPLE ON A COMMUTER TRAEN TODAY MAY HAVE BEEN RANDOM ATTACK. SURVEILLANCE VIDEO HELPED THEM CATCH THE SUSPECTED SHOOTER IN CUSTODY. ADRIENNE BROADDUS IS FOLLOWING THE STORY FOR US OUTSIDE O CHICAGO. WE HAVE SEEN A LOT OF LATE MOVING DEVELOPMENTS ON THIS. WHY DO PEOPLE THINK THIS WAS A RANDOM ATTACK? WHAT ELSE DO WE KNOW >> THAT IS THE BIG QUESTIO TONIGHT. POLICE ARE TRYING TO DISCOVER MOTIVE IN THIS SHOOTING THAT LEFT FOUR PEOPLE DEAD. HERE'S WHAT WE KNOW SO FAR INVESTIGATORS SAY THE SUSPEC WHO IS IN CUSTODY RIGHT NOW SHOT PEOPLE ON TWO SEPARATE TRAINS. TWO OF THEM DIED ON THE TRAIN. TWO WERE RUSHED TO THE HOSPITA AND DID NOT SURVIVE. THE DEPUTY CHIEF OF POLICE CALLED WHAT HAPPENED HER SENSELESS. >> IT IS NOT SOMETHING YOU WAN TO WAKE UP TO, IT IS A MONDA MORNING AND A HOLIDAY AND PEOPLE ARE SUPPOSED TO BE ENJOYING TIME OFF WITH FAMILIES. TO HAVE TO BE HERE IN SITUATION LIKE THIS IS HORRIBLE. >> Reporter: THIS ALL UNFOLDED AT 5:30 THIS MORNING IT IS LABOR DAY SO NOT AS MANY PEOPLE WERE TRAVELLING ON TH TRAIN. >> WE HEARD FROM THE MAYOR ABOUT THIS, RIGHT? >> Reporter: YES, THE MAYOR IS WEIGHING IN. WE KNOW THE SUSPECT WA INITIALLY ABLE TO GET AWAY BEFORE HE WAS DISCOVERED O ANOTHER LINE, PARTLY INVESTIGATORS SAY THEY WERE ABLE TO TRACK THE SUSPECT DOW BECAUSE OF SURVEILLANCE VIDE BUT STILL, THE MAYOR IS ASKING FOR ADDITIONAL PUBLIC SAFETY RESOURCES FROM NOT ONLY COUNTY OFFICIALS BUT AT