BRUSSELS — The attacks on the airport and a subway station in Brussels on Tuesday have intensified questions about the Belgian authorities’ ability to thwart terror in the city. This article, published on Monday, examines how the lone surviving suspect from the Paris terror attacks in November managed to elude capture for months afterward.

For 125 days, the authorities in Belgium had failed to find Salah Abdeslam, the most wanted man in Europe, though they strongly believed he was holed up in or near the insular immigrant section of Brussels where he had grown up.

They raided dozens of homes in the neighborhood, known as Molenbeek, as they hunted down Mr. Abdeslam, one of the 10 men suspected of carrying out the Nov. 13 terrorist attacks in Paris and the only one still alive. They rounded up his friends and fellow drug dealers and thieves, and interrogated members of his family.

They thought one night that they might have him cornered, but were paralyzed by a law prohibiting nighttime raids. When they moved in the next morning, he was nowhere to be seen. They deployed a drone to help in the hunt. Clues to Mr. Abdeslam’s continued presence, including his fingerprints, started to seem more mocking than tantalizing.