Belgian authorities have admitted several errors before, during and since the Brussels attacks.
Brussels had already been prepared for an attack, as several of the Paris attackers had come from Belgium and it had already been placed under lockdown weeks earlier.
And yet, security forces were often a step behind the suspects, and operational failures on the day of the bombings meant that reactions were sometimes too slow.
Why was the metro running?
The Belgian government has insisted that its response to the airport bombings on 22 March was clear. The decision to evacuate the Brussels metro and five rail stations was taken at 08:50 on 22 March, 52 minutes after two bombs went off at Brussels airport at 07:58.
But the third bomb went off at 09:11 on a metro train at Maelbeek station, in the heart of the European quarter. Metro company STIB said it received no order from the government to halt services. The second attack was well over an hour after the first and ministers are under pressure to explain what went wrong.
Rail operators too said they had been given no direction from the government. Train services continued for some time after the third bomb and operator Infrabel only put its internal crisis plan into action at 09:45.
Bomber brothers repeatedly missed
A string of errors have been made over the two brothers who blew themselves up on 22 March. But it was not just Belgian officials involved.
The case of Brahim el-Bakraoui has shone an uncomfortable light on failures in intelligence-sharing across Europe.
Both brothers, Khalid and Brahim el-Bakraoui were violent, convicted criminals freed early from jail in line with parole recommendations. Where Belgium and other countries failed, was in understanding the danger they posed as Islamists linked to so-called Islamic State (IS).
Brahim el-Bakraoui was freed in October 2014, having served four years of a nine-year jail term for a crime which involved opening fire on police with a Kalashnikov. The Belgian government has already criticised the courts for failing to keep tabs on him, He broke his parole conditions and was picked up by Turkey near the Syrian border in June 2015.
Although Turkey’s president said authorities in Ankara had warned Belgium and the Netherlands that he was a “foreign terrorist fighter”, that has been strenuously denied by the Dutch government. The Dutch argue that the Turks did not follow the steps they would normally adopt with suspects identified as radicalised Islamists.
As a result, the Dutch allowed Brahim el-Bakraoui to slip through Schiphol airport because they had no record of him. And the Belgians were still treating him as a petty criminal, rather than an Islamist, even though he had broken his parole conditions and they knew he had travelled to the Turkish city of Gaziantep, near the Syrian border.
By 25 September, Brahim el-Bakraoui had been placed on a US counter-terrorism watch list. An Interpol red notice for his brother was issued in December for his brother, Khalid.
An FBI alert about both brothers was issued on 16 March and passed on to Belgian police the following day.
Mistakes in hunt for Paris attacker Abdeslam
For four months after the 13 November gun and bomb attacks in Paris, Belgian police were searching for key Paris suspect Salah Abdeslam. Brussels went into lockdown for days afterwards amid fears of an imminent attack.
He was finally caught on 18 March, a few hundred metres from where he grew up, an event thought to have brought forward the Brussels attacks four days later.
It has since emerged that police in Mechelen, north of Brussels, had failed to pass on information as far back as early December 2015 about a radicalised accomplice, Abid Aberkan, who had been in touch with Abdeslam. He and Abdeslam were eventually caught at Aberkan’s mother’s address. Police said the source was unreliable, but acknowledged a mistake had been made.
Failings in Abdeslam interrogation
Salah Abdeslam’s capture was hailed as a major Belgian security success, and during his initial questioning he was said to have spoken freely to his interrogators. But he was questioned for only an hour between his arrest and the Brussels bombings.
A “treasure trove of information” before the attacks”, Abdeslam has since clammed up, according to his lawyer. And, one report suggests, he was only interrogated about his role in the Paris attacks, even though a detonator and weapons were found at the safe house in which he had been hiding until 18 March.
We now know that despite his arrest, at least three close accomplices – Khalid el-Bakraoui and Brahim el-Bakraoui and Najim Laachraoui – were able to blow themselves up four days later. Khalid rented the Brussels flat that Abdeslam had been hiding at until 18 March as well as the Charleroi house used before the Paris attacks. Laachraoui is thought to have been the bomb-maker.
Were clues missed before 22 March attacks?
There have been several unverified reports that Belgian authorities were warned in advance of attacks planned against Brussels airport.
According to Skai TV in Greece, a map and layout of the airport at Zaventem were found on a computer and USB stick at a flat in the Pangrati area of Athens which was used by Paris attacks ringleader Abdelhamid Abaaoud in January 2015.
Belgian prosecutors have not confirmed the allegation but the flat was searched by Greek police a few days after a Belgian raid on an Islamist cell in the town of Verviers in which two suspects were killed.
French daily Le Monde reports that handwritten notes and attack plans were found on the Athens computer hard drive – and that the details matched in every respect the bombing of Brussels airport.
Blunders in the hunt for the Brussels bombers
The search for the third bomber, wearing a hat and white jacket, has been beset with difficulties for Belgian police. Initially, there were reports that he was bomb-maker Najim Laachraoui, but then he was declared dead.
Then, a man named by Belgian media as Faycal Cheffou was arrested and accused of “terrorist murder”. He has since been freed after he said he was at home at the time, and had received calls there on his mobile phone.
His release was a major setback for investigators, who had to relaunch their appeal for help in finding the man in the hat.