Habibulloh Saipov saw his son for the first time in 13 years this week. He said Thursday that he expected that he would never see him again.

The son, Sayfullo Saipov, was convicted last month in Manhattan federal court of using a truck to kill eight people on the West Side bike path on Halloween Day 2017 and to injure many others. On Thursday, his father, allowed into the United States to testify as a jury considers giving his son the death penalty, said through an interpreter that after he finishes, he will fly back to their native Uzbekistan.

“He committed a terrible tragedy,” Habibulloh Saipov said of his son’s attack as he responded to questions posed by a defense lawyer. “He ruined their lives.”

Prosecutors opened the trial’s penalty phase last week arguing in favor of capital punishment rather than life imprisonment without any chance of release, contending Sayfullo Saipov, 35, is too dangerous for prison. Relatives of people who had died testified about the impact of the loss of their loved ones.

Thursday’s appearance by Habibulloh Saipov was part of the defense lawyers’ case against death. They had told the jury during the trial that their client had been influenced by hours of Islamic State propaganda, and that he had spent long hours viewing martyrdom videos.

David Stern, one of Mr. Saipov’s lawyers, told the jury last week that during the penalty phase, the defense would present testimony from an expert on propaganda and why ISIS targets Uzbeks, whose predominantly Muslim country is a former Soviet republic.

“You will see that his past could not have predicted his present,” Mr. Stern said. “You will hear from the family and you, like them, will wonder how in the world did this happen?”

The first Saipov relative to take the witness stand in the courtroom of Judge Vernon S. Broderick was Habibulloh Saipov. He testified that there was nothing in his Muslim faith that could justify his son’s actions.

He said Mr. Saipov, who won the immigration lottery to come to the United States, was born when the Soviet Union still controlled Uzbekistan, and there was no freedom of religion. He said he was unable to teach his son about the Muslim faith in any comprehensive way.

The defense showed the jury family photographs: Mr. Saipov’s father and mother on their wedding day; one of a young Mr. Saipov with his mother and a sister; and one of a smiling Mr. Saipov in New York. Another photo depicted a barely illuminated Mr. Saipov standing in front of a large truck.

Habibulloh Saipov described the hourslong calls that his son, who worked in the United States as a long-haul truck driver, made to him from the road, often when it was late at night in Uzbekistan.

Mr. Saipov, he said, would say, “Father, talk to me. There is a little bit until my final destination.”

Habibulloh Saipov recalled that it was the morning of Nov. 1 in Uzbekistan when he learned of the attack his son had carried out. His younger brother showed him news coverage on his phone.

“Sayfullo, Sayfullo,” Habibulloh recalled his wife saying before she fainted. She was taken to the hospital, he said. Uzbek police took him for interrogation and detained him for 15 days.

After learning what his son had done, Habibulloh Saipov said, “my soul was destroyed.”

Habibulloh Saipov said he tried to visit his son while he was in jail over the past five years but was unable to secure a visa. He was allowed to come to the United States to testify, but had to leave as soon as his testimony was complete. He was flying back to Uzbekistan on Friday, he testified.

It was then that Mr. Stern asked Habibulloh Saipov whether he expected to ever see his son in person again.

“I think after this event, I will not be able to see him,” he said, burying his face. Weeping, he said he still loved his son, “with all my heart.”

He said that a life sentence would let Mr. Saipov’s life retain meaning.

“He will get enough time to realize the thing he committed," Habibulloh Saipov said. “It’s important for his kids to know that their father is still alive.”

As Habibulloh stepped down from the witness stand, a relative — one of Mr. Saipov’s uncles — stormed out of the courtroom, striking the door and shouting in the Uzbek language.

An interpreter caught only three words: “dirty ISIS bastards.”

The judge ordered the man banned from the courthouse for the remainder of the trial.