BRUSSELS — On a cold evening in January, a few dozen Belgian police officers in tactical gear amassed outside a redbrick apartment building in the eastern town of Verviers. It had been a week since the terrorist attacks on the office of Charlie Hebdo and a kosher mini-market in Paris. Police across Europe were on high alert against additional terrorist plots, and now Belgian authorities believed they were closing in on one.
For some time, the Belgians had been monitoring the communications of two men who had holed up in Verviers. Soufiane Amghar and Khalid Ben Larbi, who were 26 and 23 years old, had disappeared from their homes in Molenbeek, a mostly immigrant, low-income neighborhood of Brussels, the spring before. Amghar’s parents reported his departure to the police, who later determined that both men had flown to Istanbul that April, and from there onto Syria.