BOSTON — One potential juror said she was biased against Dzhokhar Tsarnaev because he is Muslim. “I already feel that he’s guilty,” she said.

Another choked back tears as she said she was a neighbor of Martin Richard, an 8-year-old boy who was killed in the 2013 bombing at the Boston Marathon, for which Mr. Tsarnaev is standing trial.

A third seemed too eager to serve, as if seeking celebrity for participating, while a fourth, a biologist, told the judge he doubted he had “the personal constitution” to consider the death penalty — even for Boston’s most loathed suspect.

Jury selection in the Boston Marathon bombing case is proving to be far more complicated than anticipated: A spokesman for Federal District Court here said Thursday that the target date of Jan. 26 for opening statements was “not realistic” because many potential jurors must still be questioned about their views.