The shooting of three people at Brussels' Jewish Museum was probably a "terrorist attack", Belgian officials said on Monday, though some security experts suggested it may have been the work of a hitman rather than an anti-Semitic 'lone wolf'.
An Israeli couple and a French woman were killed in the shooting on Saturday. A Belgian man remains in critical condition in hospital. The gunman is on the run.
Police released a 30-second video clip from the museum's security cameras showing a man wearing a dark cap, sunglasses and a blue jacket enter the building, take a Kalashnikov rifle out of a bag, and shoot into a room, before calmly walking out.
A spokeswoman for the federal prosecutors told reporters the killings were being treated as a likely terrorist attack but other possibilities were not being ruled out.
"The fact that all lines of inquiry remain open means that we have no certainty about it being a terrorist attack but it seems very likely to us," spokeswoman Wenke Roggen said.
"The footage shows a person who seems cold blooded and determined," she added.
Some security experts said the way in which the assailant carried out the killings suggested planning and execution by a specialist.
Edwin Bakker, professor at the Centre for Terrorism and Counterterrorism at the University of Leiden in the Netherlands, said the gunman's calm manner indicated he had experience.
He added that no organization had claimed responsibility for the attack, suggesting it was not the work of militants.
"People use the word terrorism very quickly but when I saw the images I thought this is a hitman," Bakker said.
One of the Israeli victims, Emmanuel Riva, had previously worked for Nativ, a government agency that played a covert role in fostering Jewish immigration from the former Soviet Union, an Israeli official said.
'Leaderless Jihad' Along with Israel's foreign intelligence agency, Mossad, and its domestic security equivalent, Shin Bet, the Nativ agency was under the authority of the prime minister's office.
Miriam Riva, his wife, had also worked in the past for the prime minister's office, the official said without elaborating.
Friends of the couple interviewed by Israeli media said they both worked as accountants in government service.
Other analysts dismissed the notion that this was some kind of contract killing or settling of scores among intelligence services, saying the daylight attack indicated it was more likely a random attack on Jews.
"I don't think a professional hitman would have done it this way. It was a guy who was deranged, who had been planning on doing something like this and did it," said Robert Ayers, a former US intelligence officer.
Rolf Tophoven, an analyst at the Institute of Crisis Prevention in Essen, Germany, drew a parallel with the killing of two US airmen at Frankfurt Airport in 2011 by a young Kosovo Albanian Muslim who had been radicalized online.
"We call this leaderless jihad, people who radicalize themselves on the Internet," said Tophoven, laying out the 'lone wolf' scenario. "It would be the story of the autonomous terrorist whom nobody had on their radar screens, whom nobody knew."