Still the notion that police knew where one of the attackers was hiding and did not pounce strained the credulity, apparently, even of Mr. Geens, the country’s top justice official, who in an interview with the Flemish TV station VTM described the legal restriction against nighttime raids — dating to 1969 — as “a big handicap.” He said the police wanted to raid the home where they suspected Mr. Abdeslam was hiding right away, but could not act until after 5 a.m. which he said was “too late.”

Image Salah Abdeslam in an undated photo. Credit... Belgian Federal Police, via Associated Press

The decision to abide the law also underscored the consistent tension between security and civil liberties that governments in Europe and the United States have struggled to balance in an era of terrorist threats.

For instance, Tashfeen Malik, the Pakistani woman who with her husband killed 14 people in San Bernardino, Calif., was granted a visa to enter the United States even though she had posted a number of private messages online indicating that she had become radicalized. Federal officials said that visa officers were not permitted to look into applicants’ social media accounts because of concerns about privacy.

Days after that failed raid on Nov. 16 in the Molenbeek district, the Belgian prime minister, Charles Michel, on Nov. 19 proposed 18 new security measures in an address to Parliament, including one that would allow the police to conduct raids at any time of the homes of people suspected of involvement in terrorist activity or suspected of possessing explosives, firearms or weapons of mass destruction on the premises.

However, Parliament is not expected to take up the proposed measures before the end of January, and some lawmakers have expressed reservations about the measures because of concerns about civil liberties.

Short of more targeted measures, the Brussels region was put on the highest possible state of alert on Nov. 21, with schools, markets and public transportation closed. The Belgian authorities stepped up the frequency and intensity of the raids, although no major new intelligence has publicly emerged from those actions.

Mr. Van der Sijpt said no exceptions to the law were permitted, even after the Brussels authorities raised the city’s alert level, putting it in a state of virtual lockdown for several days. The law prohibits raids on private homes between 9 p.m. and 5 a.m., unless the person living there invites the authorities to enter, or has been directly observed committing a crime.