Mayor Bill de Blasio on Monday joined a host of prominent figures in sharply questioning how Jeffrey Epstein died in an apparent suicide in federal jail, insisting that he was not dabbling in conspiracy theories even as he echoed them.

The mayor said in an interview that he did not understand how Mr. Epstein, who was facing multiple accusations of sexual abuse and sex trafficking and who had only recently been taken off suicide watch, was left unsupervised long enough in the federal jail in Manhattan so that he was in a position to end his own life.

“It’s just too convenient,” Mr. de Blasio said. “It’s too many pieces happening simultaneously that don’t fit.”

Mr. Epstein’s death has led to an explosion of unfounded conspiracy theories on social media, often uniting people of disparate ideologies who seem to be grasping for any kind of explanation for how it could have happened.