Juan Ignacio Zoido did not elaborate on what type of measures Spanish law enforcers had introduced to pinpoint terror threats.
And he said none of those involved in the attacks on August 17 and 18 had raised suspicion previously.
But he said: “It’s true that these controls exist.
“It’s possible some checks weren’t made in this case and we will have to determine how we can avoid this happening again.”
Muslim cleric Albdelbaki Es Satty, 42, who is suspected of radicalising the group, was monitored by authorities after spending two years in prison for drug trafficking.
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On his release, the Moroccan became the Imam of a mosque in Ripoll, a small town in the foothills of the Pyrenees 65 miles north of Barcelona.
But a judge decided he did not pose a “serious threat” and blocked attempts to deport him.
He went on to radicalise local youths, mostly Moroccan immigrants aged 17 to 34, and is thought to have been the leader of the 12-man cell who brought mayhem to the region last month.
Es Satty and another cell member died on August 16 when gas canisters being stored in the terrorist hideout accidentally blew up, destroying the house and the jihadi’s sinister bombing plan.
Police believe the remaining perpetrators were then forced to abandon plans to bomb Barcelona’s top tourist attractions and opted to carry out ramming and knife attacks instead.
Mr Zoido also rejected claims an opportunity to foil the Barcelona and Cambrils plot shortly after the Alcanar blast was lost due to poor cooperation between the Catalonia police and the national police.
He stressed that national and Catalan security forces had worked closely together, but added that “a very meticulous analysis” of their communications will still be carried out.
Catalan police initially suspected that the explosion was a result of a gas leak or an accident at a drugs factory.
Sources said the bomb squad was sent to Alcanar 10 hours after the blast but information was not passed to the national police which has more anti-terrorism experience.