Two other suspects, Ibrahim Abdeslam and his brother Salah, were interrogated and released by the Belgian authorities after trying to travel to Syria earlier in the year. A fourth man suspected of involvement in the attacks, Bilal Hadfi, lived in Brussels, but reports earlier this year of his radicalization at school and rumors of his departure to Syria were not passed on to the police, according to school officials.

After the attacks, near misses by the Belgian authorities and their French counterparts continued. Salah Abdeslam was involved in a routine traffic stop by the French police the day after the attacks but was allowed to continue on because he had not yet been linked to them. Adding to the string of missteps, he may have managed to evade a huge manhunt because of a Belgian law banning police raids on private homes during certain hours.

On Wednesday, police officials confirmed that an investigation into the reports of an orgy among police officers had begun.

“I cannot give more information on the internal investigation, because it is ongoing and because investigations into the Brussels police force are highly complicated,” the Brussels-West police spokeswoman said in a phone interview. “Right now, we don’t know the extent of the investigation yet. Maybe officers from other zones are implicated, and I don’t want to set off an internal fight by commenting on police officers from other zones. Internal investigations into the Brussels police are, as a rule, highly complicated and very sensitive.”

The fractured nature of policing in Brussels has been a target of criticism since the Paris attacks. Brussels, while officially bilingual, is largely French-speaking. However, it is in the country’s Dutch-speaking region, which has gained wealth and clout in recent decades relative to Wallonia, the French-speaking region to the south. (There is also a tiny German-speaking minority in the southeast.)

Power in Belgium is delicately distributed along regional and linguistic lines, making political oversight difficult, not to mention basic intelligence-sharing.

In addition to the six local police agencies, Brussels has a federal police service, two intelligence services — one military, one civilian — and a terrorism threat assessment unit whose chief, exhausted and demoralized, resigned over the summer but remains on the job.