Belgium Rues Missed Terror Signs

European counterterrorism agencies have been on high alert since the November attacks in Paris, where the radical group proved it could strike hundreds of miles away from its strongholds in Syria and Iraq. Using Belgium as a staging ground, the group deployed operatives who were deft at crossing borders and blending in with the masses.

Then, like a modern-day Hydra, Islamic State militants regrouped successfully to plot the Brussels attacks right under the noses of the authorities, who are now ruing the missed signals. The attackers went back and forth between multiple hide-outs and a bomb laboratory with chemicals stockpiled in industrial proportions, at precisely the same time police were combing the neighborhood in search of militants tied to the Paris attacks.

Belgian authorities are now under growing pressure to explain a string of missteps that depict a chaotic counterterrorism apparatus unable to process even basic information and keep track of dangerous criminals.

In one of the starkest examples, local Belgian police in the town of Mechelen said Friday they had the address of a possible safe house of alleged Paris attacker Salah Abdeslam in their files since December but didn’t upload the information into a national police database.

Mr. Abdeslam, allegedly the logistical chief of the operation in Paris, which killed 130 people, was captured March 18 at that address after eluding Europe’s security dragnet for four months.

The Belgian government has also admitted to missing an opportunity to arrest one of the three men named as Brussels suicide bombers, Ibrahim el-Bakraoui.

When Turkey informed Belgium it had apprehended Mr. Bakraoui near the Syrian border last summer, Belgian authorities didn’t seek his arrest or list him as a potential terrorist, even though Mr. Bakraoui was a convicted criminal who had violated his parole. While the two countries spent eight months debating his significance, Mr. Bakraoui flew to the Netherlands as a free man and vanished.

The other two suicide bombers in Brussels, Najim Laachraoui and Khalid el-Bakraoui, Ibrahim’s younger brother, were known by police for their alleged implication in preparations for the Paris attacks. Both men were targets of international arrest warrants on terrorism charges.

The police and judicial failures have triggered a political crisis in Belgium and are a chilling reminder how Brussels has become a hub for militants planning attacks in Europe.

“If you put all facts together, you can indeed put a lot of question marks with what happened at the justice department and police,” Belgian Justice Minister Koen Geens said earlier this week.

Mr. Geens and Interior Minister Jan Jambon offered to resign, though Belgium’s prime minister asked both men to stay in office.

The Paris attacks have exposed wider intelligence breakdowns across Europe, which allowed dozens of militants trained or inspired by Islamic State to crisscross the continent, often with fake documents.

“Counterterrorism agencies are drawn in front of an immense threat,” said Claude Moniquet, a security consultant in Belgium and former intelligence officer in France.

The first sign that Mr. Laachraoui, one of the Brussels suicide bombers, might have radicalized came slightly more than three years ago through a phone call.

Mr. Laachraoui was a Belgian national born in Morocco in 1991. In February 2013, he briefly contacted his parents and said he had reached Syria.

The family informed Belgian police, according to Mourad Laachraoui, a world-class taekwondo fighter and the younger brother of the suicide bomber.

At the time of the tip from Mr. Laachraoui’s family, Sunni insurgents in Syria and Iraq hadn’t yet announced the creation of the Islamic State caliphate. Their ability to lure foreigners was widely overlooked in European capitals.

Belgium’s justice and interior ministers, said at a parliamentary hearing Friday that Turkish authorities informed a Belgian liaison officer that they had detained Ibrahim el-Bakraoui in the Turkish town of Gaziantep on June 11.

Gaziantep is about 10 miles from the Syrian border and has been one of the gateways used by foreigners to reach Islamic State territories. Three days later, the officer passed on the information to Belgium’s counterterrorism unit. The officer’s name hasn’t been publicly disclosed.

The counterterrorism unit replied with a request for more details on why Turkey had detained the Belgian national, the two ministers said.

For the next eight months, Belgian and Turkish officials grinded away in a lengthy correspondence focused on what kind of threat Mr. Bakraoui represented. A formal answer, according to the Belgian ministers, only came in January of this year, when Turkey told the liaison officer that Mr. Bakraoui was suspected of links with Islamic State.

By then, Mr. Bakraoui had disappeared. On July 14, he boarded a flight to the Netherlands. Neither Dutch nor Belgian authorities sought to detain or question him.

Belgium’s justice and interior ministers said earlier this week that Belgian authorities had grounds to arrest Mr. Bakraoui because he had been violating his parole since May. But his name wasn’t added to Belgium’s national list of fugitives until the end of August, the ministers said.

At Friday’s hearing, Mr. Jambon, the Belgian interior minister, blamed the liaison officer for his lack of diligence.

“We can’t but conclude that not a service, a direction, but one person has been negligent,” he told lawmakers Friday. “A convict who spends years in prison, then goes to the Turkish-Syrian border…You don’t have to be familiar with terrorism for a long time to see that the chance is 90-95% that you are dealing with a foreign fighter.”

It isn’t clear why Belgium’s counterterrorism unit didn’t draw the same conclusion.

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