Mr. Zerkani, a Moroccan who became a Belgian resident in 2002, moved on the fringes of Mr. Ayachi’s milieu but did not attract close attention, people in his neighborhood said. While the Islamic Center had championed an ascetic and rigid form of Islam that had only modest appeal to young people who liked to drink alcohol and carouse at night, Mr. Zerkani, Belgian investigators said, was able to bridge the divide by channeling the criminal energies of young delinquents.

Image Abdelhamid Abaaoud, a Molenbeek resident who the authorities say commanded the November attacks in Paris, was believed to have been an associate of Mr. Zerkani. Credit... via Agence France-Presse — Getty Images

Among these Zerkani recruits, the investigators said, were Mr. Abaaoud, who was killed a few days after the Paris attacks when French police officers stormed his hide-out, and Salah Abdeslam, a former drug dealer and onetime bar owner suspected of being an architect of the Paris attacks, who was captured in Molenbeek last month after four months on the run.

Mr. Zerkani was born in Zenata, an area of northern Morocco inhabited by the country’s often rebellious Berber-speaking minority, and spent time in Spain and the Netherlands before moving to Belgium when he was 28. In Brussels, his associates included supporters of the Shabab, a militant group in Somalia affiliated with Al Qaeda. Belgian investigators say he shoplifted and committed other petty crimes before becoming a prolific recruiter for the Islamic State.

A senior Belgian terrorism investigator, speaking on the condition of anonymity because he was not authorized to do otherwise, said that it was not clear how Mr. Zerkani had become a trusted operator for the Islamic State and its predecessors, and that the extent of his involvement with people connected to plots was only now coming into focus.

One theory is that Mr. Zerkani was recommended to the militant group’s leadership in Syria by Fatima Aberkan, a friend of the wife of the Qaeda militant who killed Mr. Massoud, the Afghan warlord. In the 2015 trial that featured Mr. Zerkani, a Belgian court sentenced Ms. Aberkan to five years in prison for participation in a terrorist organization; she and five of her children, three sons and two daughters, have spent time in Syria with the Islamic State, prosecutors said.

There is no record of Mr. Zerkani traveling to Syria. But prosecutors said that at least 18 people he was in regular contact with went there to fight between mid-2012 and 2014.