The bomb attacks in Belgium offer new evidence of its security forces’ shortcomings in monitoring violent Islamist radicals, a failure that has allowed this country at the heart of Europe to become an incubator of terror.

One glaring example: Belgian authorities knew that at least one of the two siblings who blew themselves up in Tuesday’s attacks — Ibrahim el-Bakraoui, 29 — had entered Turkey with the apparent intent of joining Islamist militants in Syria, according to a senior Turkish security official, who spoke on the condition of anonymity because of the sensitivity of the subject. Bakraoui was stopped by Turkish authorities last summer at the Syrian border and sent to the neighboring Netherlands. But Belgian officials now say that at the time, they did not equate his attempt to join the fighters with a possible terrorist threat.

His younger brother, Khalid el-Bakraoui, 27, was also monitored in Turkey, but it remains unclear what the Belgians knew about his movements. Eventually, Khalid el-Bakraoui became a logistical operative in the Paris siege that left 130 people dead in November, according to a senior European official familiar with the case. He then joined his brother in the coordinated attacks Tuesday that killed 31 people and wounded more than 270 at the Brussels airport and in the city’s metro system. The Islamic State asserted responsibility for both attacks.

The missed cues did not stop there. A Belgian citizen of Moroccan descent, Najim Laachraoui, 24, was identified Wednesday as another of the suicide bombers at the Brussels airport, and he is said by authorities to have been involved in crafting the deadly explosives used in the attacks. His DNA was found on explosives detonated in the November assault in Paris. That suggests that Laachraoui had managed not only to elude capture in recent months but also to operate on Belgian soil.

What we know about the connections between the Brussels and Paris attacks

Belgium’s failure to head off Tuesday’s attacks comes 4 1 /2 months after authorities realized that the Paris attacks had been largely plotted out of Molenbeek, a working-class, heavily North African neighborhood in Brussels that is considered a breeding ground for violent young jihadists. But the latest revelations suggest that despite dozens of raids and detentions across Europe, the network behind the France plot was never fully dismantled, leaving some of its members to act again.

The problems in Belgium include anti-terror operations that failed to capture their targets. For example, officials say at least one of the Belgian brothers, Khalid el-Bakraoui, had served as a frontman in leasing safe houses for the Paris attackers and their alleged accomplices, including Salah Abdeslam, Europe’s “most wanted man,” who was arrested last week by Belgian authorities. But two suspects escaped a raid on one such safe house in Brussels three days before Abdeslam’s capture. Analysts say that the suspects who fled could have included one or both of the Bakraoui brothers. Fresh fingerprints of Abdeslam were found in the apartment.

“If that operation would have been planned differently, the suspects would not have escaped via the roof of that apartment,” said Jean-Charles Brisard, chairman of the Center for Analysis of Terrorism in Paris. “This is crazy. This is something that should never have happened.”

Yet the troubles in Belgium run far deeper than intelligence lapses or bungled operations. This multilingual nation — in which citizens speak French, Dutch and German — is plagued by societal rifts and rivalry between jurisdictions. Belgium has undergone radical devolution since the 1980s in which federal power has been increasingly concentrated in a complex array of local and communal bodies.

Brussels has six police forces, each answering to a different mayor. Information is not always shared between the agencies, making it easy for leads to fall through the cracks.

Like Abdeslam, terrorist plotters have managed to hide in plain sight in Brussels. That is partly a result of open borders that, until the Paris attacks, allowed a free flow of people throughout much of the European Union. Co-conspirators could travel between Belgium and France without attracting authorities’ notice.

Brussels continues to morn the victims of bombing attacks that killed 31 people and wounded more than 270. But people are expressing a resolve not to let the fear of terrorism rule their lives. (Jason Aldag/Cléophée Demoustier for The Washington Post)

But the terrorists’ relative ease of movement, critics say, also reflects the failings of a convoluted and overwhelmed law enforcement system in Belgium.

Although this nation of 11.2 million has sent more foreign fighters per capita to the Islamic State than any other country in Europe, Belgium has a relatively small security apparatus. Brussels, the capital, is home to 2,500 international agencies and organizations, including NATO and the E.U. headquarters. Yet nationwide, the Belgian federal police have a total force of approximately 12,000.

The Belgian police have also been hampered by bizarre rules. According to Belgian Justice Minister Koen Geens, just two days after the Paris attacks Abdeslam was “likely in a flat in Molenbeek.” But because of the country’s penal code, which prohibits raids between 9 p.m. and 5 a.m. unless a crime is in progress or in case of fire, police were ordered to wait until dawn to pursue him. By then, Abdeslam was nowhere to be seen.

Despite warnings that Belgium could be a target of terrorist attacks, security at the Brussels airport was inadequate. In the beginning of this year, a Belgian union expressed alarm at findings of tests run at the airport to detect bombs in carry-on luggage. In one round of tests, half the bombs were not detected, according to Christina Schori ­Liang, a senior fellow at the Geneva Center for Security Policy.

The inspectors also revealed that fences around the airport had holes that were not repaired for months, Liang said. The ­security-clearance process was found to so be lax, she added, that employees could begin working without waiting for the process to conclude. It can take up to three months.

On Wednesday, Turkish President Recep Tayyip Erdogan, whose country has been a transit point for young Europeans drawn to the Islamic State, said one of Tuesday’s attackers — he did not say which — was arrested in his country last summer and deported back to Europe.

The Turkish official said the president was referring to Ibrahim el-Bakraoui, who was stopped at the Syrian border. Asked whether Belgian officials were notified, the official said, “Yes, they knew.”

On Wednesday, Geens, the justice minister, confirmed that officials were aware that Ibrahim el-Bakraoui, who had a record of violent crime, had been expelled by the Turks to the Netherlands after trying to enter Syria. Speaking to a local radio station, he declined to say whether officials knew he had reentered Belgium. But he seemed to suggest that the attempt to enter Syria did not indicate a special threat.

“At that moment, he was not known for terrorism, but as a criminal,” Geens said.

Analysts say Belgium’s terrorism problem goes beyond security issues and includes social divides related not only to linguistic barriers but also to incorporating waves of Muslim immigrants in recent decades. Immigrants and their children maintain that they are ostracized and find it more difficult to get jobs.

Belgian youth born outside the European Union had an unemployment rate of 43.6 percent in 2014, compared with a rate of 23.2 percent for Belgian-born youth.

Missy Ryan and Annabell Van den Berghe contributed to this report.

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