Federal prosecutors appealed to the public for information about 24-year-old Najim Laachraoui, who allegedly traveled to Hungary with Abdeslam before the Nov 13 carnage, and has been traced to safe houses under a false name.
Laachraoui was checked by guards at the Austria-Hungary border on Sept 9 while driving in a Mercedes with Abdeslam and one other person, Belgium's federal prosecutors said in a statement.
Laachraoui, whose nationality wasn't disclosed, had traveled to Syria in February 2013, prosecutors said. It wasn't clear when he returned to Europe.
Laachraoui is "someone who must explain himself," Belgian prosecutor Frederic Van Leeuw said, stressing that "clues" don't amount to proof.
Investigators have struggled with the complexity of the Paris attacks case.
"We are far from putting the puzzle together," Van Leeuw said. He stressed that the public should come forward with information they may have.