Belgian authorities have held police raids in Brussels and two other Belgian cities and kept four of the people they picked up under detention.

The federal prosecutor's office says the raids are linked a "federal case regarding terrorism" but did not specify whether the case had anything to do with the March 22 suicide bombing attacks in Brussels that killed 31 people and left 270 wounded.

The 13 raids in Brussels and the northern cities of Mechelen and Duffel came early Sunday. An investigating judge was to decide later Sunday whether the four new suspects will remain in custody. Five others were released after questioning.

'There have been errors'

Tuesday's bomb attacks are also tearing at the fabric of Belgium's government, justice system and police, and the country's interior minister sought Sunday to contain the growing criticism of the government's handling of the tragedy.

Interior Minister Jan Jambon conceded Sunday that decades of neglect had hampered the government's response to violent extremism. He said the government has invested €600 million ($890 million Cdn) in police and security services over the past two years but Belgium's justice system and security services are still lagging behind.

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The 13 raids in Brussels and the northern cities of Mechelen and Duffel came early Sunday. (Google/CBC)

Jambon, whose offer to resign Thursday was declined by Belgian Prime Minister Charles Michel, also acknowledged some shortcomings prior to the attacks.

"There have been errors," he said on VRT television.

Jambon said it takes time to hire anti-terror specialists and specialized equipment and insisted that the government's new investments need time before they become visible to the public.

As international pressure on Belgium has mounted for serving as an unwitting rear-base for extremist fighters who launched the Nov. 13 massacres that left 130 dead in Paris, the government has felt forced to defend its choices and the actions of investigators. Lawmakers, meanwhile, are demanding an inquiry.

Belgian police and the army have been deployed, sometimes around the clock, at major buildings and sites in the capital in increasing numbers since November, when Brussels went into lockdown over fears that top Paris suspect Salah Abdeslam had returned and was hiding there.

As it turned out, Abdeslam did return, but police did not find and arrest him until March 18, four days before suspects from his network exploded suicide bombs in Brussels.

Why Belgium is a target for attacks3:40

Belgian investigators have been slammed for not questioning Abdeslam long enough or hard enough after he was shot in the leg during his arrest. Police have also been criticized for taking too long to get to Zaventem airport on Tuesday morning after two suicide bombers blew themselves up there — and left an even bigger third suitcase full of explosives that did not go off.

Jambon and Justice Minister Keen Goens were grilled by lawmakers Friday over how authorities failed to arrest suicide bomber Ibrahim El Bakraoui before he blew himself up in the packed departure hall at Brussels Airport.

Turkey has said that Bakraoui — whose brother Khalid was the suicide bomber at the Maelbeek subway station on Tuesday — was caught near Turkey's border with Syria in 2015, and Ankara had warned Brussels and the Netherlands that he was "a foreign terrorist fighter." Belgian authorities said they did not know he was suspected of terror-related activities until after he was deported to the Netherlands.

Experts discuss the attack in Brussels6:45

Jambon also said the Brussels subway network had been told to shut off services around 20 minutes before the attack at the subway station, which is close to both the European Union headquarters and the U.S. embassy. He did not fully explain why it was not closed in time, raising more questions about the efficiency of Belgium's security services.

ID forger arrested in Italy

On Sunday, Italian police in the southern city of Salerno said they had arrested an Algerian wanted in Belgium for an alleged false ID crime ring. Djamal Eddine Ouali was arrested Saturday in the town of Bellizzi, said Luigi Amato, the head of Salerno police's anti-terrorism squad. Ouali, 40, was being held in jail while authorities expect extradition procedures to soon begin.

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Belgian prosecutors said Sunday Djamal Eddine Ouali is thought to have made false documents for some of the attackers in the Nov. 13 massacre in Paris. (Italian police/Reuters)

Ouali came to Italy's attention by applying for a residency permit. Police noted Sunday he had the same name as a man sought by Belgium for alleged involvement in "a criminal network dedicated to false documents on a large scale" that emerged last year in Brussels.

Italian police are investigating why Ouali was in Italy and expect extradition procedures to begin.

Belgium issued a warrant for Ouali in January, three months after a raid in the Brussels suburb of Saint-Gilles yield about a 1,000 digital images used for false documents.

Belgian prosecutors said Sunday that Ouali is thought to have made false documents for some of the attackers in the Nov. 13 massacre in Paris, including Salah Abdeslam. Investigators are trying to establish whether the same false ID ring provided papers for the March 22 attackers.