“The evidence that had led to the arrest of the man named Fayçal C. was not substantiated by the evolution of the ongoing investigation,” Thierry Werts, another spokesman for the federal prosecutor, said in a statement. “Consequently, he has been freed by the investigative judge.”

Belgians writing on Twitter in Dutch expressed outrage over the latest turn of events. “We got him but it was the wrong guy #painful,” said one. Another fumed: “Cheffou the new hero of the Left on Twitter. I’m going to be sick.” Others, mostly writing in French, pilloried the authorities for having arrested Mr. Cheffou in the first place.

Mr. Cheffou had been picked out of a photographic lineup by a cabdriver who shuttled three men to Brussels Airport, where two of them — Ibrahim el-Bakraoui and Mr. Laachraoui — blew themselves up at 7:58 a.m. last Tuesday. Mr. Bakraoui’s younger brother, Khalid, blew himself up at 9:11 a.m. at the Maelbeek subway station.

The death toll from the attacks rose on Monday to 35, as the authorities reported that four victims who had been hospitalized died from their injuries. The toll, which was reported by the Belgian health minister, Maggie De Block, did not include the three suicide bombers.

In an interview on Sunday, Yvan Mayeur, the mayor of the City of Brussels, the central borough of the 19 municipalities that make up the Belgian capital, said that the taxi driver had identified Mr. Cheffou in a police photo lineup, but that the authorities were still waiting for DNA confirmation that he was the bomber.

Mr. Mayeur added that he did not know whether Mr. Cheffou had been involved in terrorism, but did know him to be a local troublemaker who had repeatedly disrupted a camp of refugees in Parc Maximilien, near the Gare du Nord railway station.