In the first phase of the federal trial, Mr. Bowers’s defense team did not present any witnesses, and never disputed the essential facts of the attack: that on Oct. 27, 2018, he drove to a synagogue where three congregations were meeting for services — Tree of Life, New Light and Dor Hadash — and walked through the building shooting worshipers.

The victims killed were Joyce Fienberg, 75; Richard Gottfried, 65; Rose Mallinger, 97; Daniel Stein, 71; Melvin Wax, 87; Irving Younger, 69; Dr. Jerry Rabinowitz, 66; the couple Bernice, 84, and Sylvan Simon, 86; and the brothers Cecil, 59, and David Rosenthal, 54.

Six more people were wounded, including four police officers.

Mr. Bowers was found guilty on 63 counts, including hate crimes that carry a maximum sentence of death. The central question facing jurors over the last two and a half weeks was whether Mr. Bowers intended to kill his victims — one of the factors necessary for a death sentence. Mr. Bowers’s attorneys called a series of experts in psychology and neurology to testify, in an effort to make a case that severe mental disease made him incapable of forming a conscious intent to kill.

“The issue in this case is, what happens when your brain is broken?” said Michael Burt, a defense attorney, in his closing argument. “What happens when you don’t have the ability to know what is truth and what is not truth?”

Defense witnesses who had examined Mr. Bowers said he had schizophrenia and other serious mental disorders. They testified that he had signs of “permanent brain damage,” that he suffered from paranoia and delusions, and that his assertion that he was a savior of the white race was so divorced from reality that it showed him to be “blatantly psychotic.”