The inquiry over the shooting, which killed three people outright and left a young man brain dead, was initially opened for murder and attempted "murder with premeditation."
Yesterday, the inquiry was transferred to the federal public prosecutor's office authorised to handle terrorist cases and spokeswoman Wenke Roggen said a probe had been opened into "terrorist" assassination.
Police analysis of images from the surveillance cameras at the museum show "a man killing in cold blood ... With great determination," Roggen said.
Earlier, deputy public prosecutor Ine Van Wymersch said the decision to transfer the case was based on "the identity and nationality of the victims".
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The victims included an Israeli couple in their 50s said to be working for the government, Miriam and Emmanuel Riva.
He was said by Belgian and Israeli news reports to have once worked for Nativ, a government agency that played a covert role in fostering Jewish immigration from the former Soviet Union.
All four victims of the shooting, which included a French woman who did volunteer work at the museum and the 24-year-old Belgian museum employee said to be brain dead, were hit by bullets to the face and neck.
The young Belgian was said by Jewish leaders to have died yesterday of injuries in the Saturday afternoon shooting but Van Wymersch said he was still alive but "clinically dead."
Three chilling security camera videos show the gunman, wearing a cap and sunglasses, but with his features hard to make out, walk into the museum entrance, remove a Kalashnikov-style automatic rifle from a bag and then shoot through a door before making an exit.
The Derniere Heure tabloid today quoted a source close to the inquiry as saying: "We fear a new Merah.