Two men have been charged with crimes related to terrorism in connection with Tuesday's attacks on Brussels. Meanwhile, a march planned to commemorate the attack and express national unity has been canceled because of security concerns.

The attacks, which killed 31 and wounded more than 300, struck Brussels Airport and a metro station in the city. The suicide bombings have been claimed by the Islamic State.

The men charged Saturday were arrested during a series of sweeps and raids conducted by Belgian authorities on Thursday and Friday.

One of them, identified as "Faisal C," was charged by an investigating judge with "leading participating in the activities of a terrorist group, terrorist murders and attempted murders." That man's house was searched, the Brussels Prosecutor says, but no weapons or explosives were found.

The other man, "Aboubakar A," has been charged with "leading participation in activities of a terrorist group."

As the investigation continues, the unofficial "march against fear" planned for Brussels on Sunday has been called off, NPR's Eleanor Beardsley reports.

The nation's interior minister appealed to the public on Sunday asking for them not to rally, the Associated Press says, saying "we understand fully the emotions" but that police were stretched too thin as they worked to trace the terrorist network behind the attacks.

Three of the suspected suicide bombers have been identified by authorities as brothers Ibrahim El Bakraoui and Khalid El Bakraoui and suspected Paris attacks bomb-maker Najim Laachraoui.

At least one other attacker is believed to have been involved. The "man in the hat" — seen with Ibrahim El Bakraoui and Laachraoui on a widely-distributed still from a security camera in the airport — has not yet been publicly identified by police. Some Belgian media outlets have speculated that he is the recently-detained man identified as "Faisal C."

There are also reports in French and Belgian media that police suspect a second attacker may have been working with Khalid El Bakraoui in the attack on the metro station. Authorities have not confirmed if they are looking for an accomplice.

In the wake of the attacks, Brussels' counterterrorism efforts have been criticized.

Broadly speaking, the divisions between Brussels' French-speaking and Dutch-speaking regions hamper coordinated efforts, NPR's Eleanor Beardsley reported earlier this week: there are two intelligence agencies and six police zones in Brussels, Eleanor says, a city of just a million people.

There are also specific shortcomings being called out in these attacks. Salah Abdeslam, the chief suspect in the Paris attacks, was arrested in Brussels days before the attacks on the airport and metro. Counterterrorism officials have told NPR's Dina Temple-Raston that Abdeslam may have been planning to be a part of the Brussels attack.

Some critics say Belgian investigators failed to push hard enough to get Abdeslam to provide information about upcoming attacks.

Authorities also initially said they had no knowledge of any ties between the Bakraoui brothers and terrorism, but have since admitted that the two brothers were on a terrorism watchlist in the U.S.

Then Turkey's President Recep Tayyip Erdoga announced that Turkey deported Ibrahim El Bakraoui to the Netherlands in 2015, and told Belgian authorities he had ties to terrorism — but that Bakraoui was released anyway.

Two Belgian officials, the ministers of justice and the interior, admitted on national television that Bakraoui should have been arrested after violating his parole by traveling. They attempted to resign over the government's failure to prevent the attacks; the prime minister refused to accept their resignations.

Meanwhile, police in France said Thursday that they foiled a would-be terrorist plot in Argenteuil, a Paris suburb.

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