He appeared to have been been hiding in Belgium for four months, sheltered by friends and accomplices.
And when he was arrested last Sunday police found weapons and evidence that more attacks were being planned.
To read our coverage of the November Paris attacks and their aftermath click here
“We feared a terrorist attack and it happened,” said Belgian Prime Minister Charles Michel on Tuesday. “At the airport Zaventem and in a metro station in Brussels, terrorists committed murder and we are being confronted with a situation with many dead.”
Authorities pushed the level of national alert up to is highest, meaning increased security measures such as stopping public transport and border controls.
Brussels was placed on high alert last year as well, after the Paris attacks, when attackers were tracked down to the Brussels suburb of Molenwijk.
Then a lot of finger-pointing started and is still going on.
French politicians have linked Belgian security lapses to the Paris attacks and Abdeslam's escape.
Belgians themselves find it harder and harder come to grips with what happened.
“Proportionally we have the most fighters present in Syria. So it is very clear, we underestimated it, we discovered it much too late this phenomenon,” says terrorism expert Bruce de Ruyver, of the University of Gent.
From the outset security services did not see what was right under their noses, he told RFI.
“We reacted too late to certain particular groups like Sharia4Belgium and to radicalised youth who went off to Syria," he claims. "When some of them returned we were not prepared, we didn’t know what to do with these people.
Those who had committed crimes were convicted, de Ruyver points out, and others were placed under surveillance.
“I think, given the exceptional situation, we did our best but for sure not enough in that sense that you can't deny that some of them escaped and that they were capable of doing things which were by far not expected by those who were controlling them,” he says.
After the attacks in Paris several raids took place in Brussels and other towns. Molenbeek was labelled a hotbed for jihadists in the press.
It was there that Salah Abdeslam was caught.
One of the theories as to why Islamists choose the distritct to establish safe houses, is that it straddles the Flemish and French parts of Brussels. The policehave poor relations, the argument goes, and do not share information in a systematic way, use different transliterations for Arab names and sometimes do not even communicate.
“Never before were authorities in these regions were confronted with such a situation,” says de Ruyver.
“I can imagine that you have gaps in your information flow. And this explains a little bit why some of prospective terrorists just stay under the radar and were not traced in time. And it is for sure that the cooperation between the local task forces in terms of information flow and the intelligence service and the special police force dealing with terrorism was perhaps not perfect."
Another problem is that local security services have few staff fluent in Arabic.
And it is still unclear how much support the terrorists have from other citizens or what can be done about it if they have.
There were reports of local youth throwing stones at police in Molenbeek when they moved in to arrest Abdeslam.
And it is not clear if that was a show of support for him or simple hatred of the police.
There are now calls in Belgium for close scrutiny of areas like Molenbeeck, more and perceived as the cradle of European terrorism.