The federal trial of the gunman who killed 11 worshipers at a Pittsburgh synagogue, the deadliest antisemitic attack in the nation’s history, began on Tuesday with a minute-by-minute description of how the massacre unfolded on a chilly October morning in 2018.

Soo C. Song, one of the lead prosecutors, began her opening statement by describing how each of the victims arrived at the synagogue on Oct. 27, “in the sanctuary and refuge of their holy place.” The 22 people at the synagogue that morning, half of whom would be killed, were from three different congregations: Tree of Life, New Light and Dor Hadash. Ms. Song described them greeting other worshipers at the door, chatting casually in the kitchen and sitting in the pews for prayer.

She then spoke of the defendant, Robert Bowers, describing his flurry of hate-filled postings on social media and how, at the same moment that the worshipers were gathering for services, he was “making his own preparations to destroy, to kill and to defile.”

Prosecutors are seeking the death penalty for Mr. Bowers, 50. This stage of the trial will take place in two parts. The first, which began Tuesday, concerns guilt; if Mr. Bowers is found guilty, proceedings will follow to determine whether he receives a death sentence.

The facts surrounding the shooting are mostly undisputed, so the trial will effectively be a monthslong assessment of whether the defendant should be executed. Mr. Bowers’s lawyers have offered to resolve the case with a guilty plea on all counts, in exchange for a sentence of life in prison without the possibility of release, but federal prosecutors have rejected these offers.

After Ms. Song spoke for roughly 40 minutes, Judy Clarke, a lawyer with a long record of defending people accused of capital crimes, delivered the opening statement for the defense.

Ms. Clarke began by saying there was “no disagreement” that Mr. Bowers was the person who killed the 11 congregants that morning, calling the killings an incomprehensible tragedy. She also acknowledged that Mr. Bowers had made “reprehensible” comments online.

But she said that, unlike a state trial, which might turn on a “straightforward” question of whether a defendant had committed murder, many of the 63 charges in the federal trial required a determination of motive and intent.

And while Mr. Bowers had told police at the scene of the shooting that he had committed the murders because he believed Jews were “killing our people,” Ms. Clarke argued that such statements were signs of his “irrational motive and his misguided intent.”

In rulings on motions by the defense and the government, U.S. District Judge Robert J. Colville has limited what can be discussed in the guilt phase of the trial. Ms. Clarke said that much of what the defense team intended to present about Mr. Bowers’s background would not come up in this part of the trial. His defense lawyers have said in motions that he suffers from schizophrenia and other mental illnesses.

In the government’s opening statement, Ms. Song described how Mr. Bowers, armed with a semiautomatic rifle and three handguns, “moved methodically through the synagogue to find the Jews he hated so much and to shoot them and kill them.” She emphasized that he did not spray the chapels with gunfire, but rather had shot six of his 11 victims in the head, two at extremely close range.

Ms. Song warned the jury that prosecutors would present gruesome evidence and descriptions of the extent of the violence that day, but she said that such details were the only way to show “the depths of the defendant’s malice and his hate.”

Witness testimony in the trial began after the opening statements were completed. Prosecutors played recordings of two 911 calls made during the shooting by Bernice Simon, 84. She was killed along with her husband Sylvan in the same chapel where they had gotten married decades earlier.

“We’re being attacked,” she was heard crying into the phone, with the sound of shots audible in the background. “My husband’s bleeding! My husband’s shot!”

The dispatcher told Ms. Simon to cover her husband with a sweater and to check his pulse. Then the gunman reappeared, and the dispatcher told Ms. Simon to stop talking. The recording of the call erupted in an explosion of noise. Then it grew silent.

“Stay quiet for me, Bernice,” the dispatcher said, to no answer. “Are you still with me?”

Jon Moss contributed reporting.