Authorities in Belgium are on the hunt for a possible accomplice to Mehdi Nemmouche, the man alleged to have carried out a shooting rampage at the Brussels Jewish Museum in May 2014, Reuters reports.

The attack resulted in the immediate death of three people: an Israeli couple and a French woman. One further individual, a Belgian man who worked in the museum, died later in the hospital due to wounds sustained during the attack (see Brussels Jewish Museum Shooting Leaves Four Dead and Man Shot in Brussels Jewish Museum Attack Dies From Wounds).

Belgian Police claim that four days after the attack was carried out, a yet-unidentified man was seen walking with Nemmouche by the Brussels North train station. Nemmouche is currently in Belgian custody (see France Will Extradite Alleged Brussels Jewish Museum Shooter).

A spokesperson who spoke to Reuters did not elaborate on how they believed this possible accomplice might be linked to the attack. He would only say, “We want this person to explain his presence,” at the train station with Nemmouche.

The announcement comes amidst a crackdown on Islamic extremists and alleged terror cells across Belgium. It follows the deadly attack on the offices of satirical French newspaper Charlie Hebdo. Raids on terror cells in Belgium in the past week have resulted in the death of two alleged jihadis and the arrest of at least 13 others.

A Belgian museum that had been planning a tribute exhibition for Charlie Hebdo‘s slain cartoonists elected to cancel the show due to security concerns (see Belgian Museum Cancels Charlie Hebdo Exhibition).