ALEXANDRIA, Va. – The grandfather of a two year-old victim of the September 11, 2001, attacks, testifying at Zacarias Moussaoui’s sentencing trial yesterday, described watching on television as the plane carrying his son and granddaughter hit the World Trade Center.

The wrenching first-person account of the day’s horrors came on the same day the judge in the death-penalty trial warned prosecutors against relying too heavily on such emotional testimony to influence Moussaoui’s jury.

The grandfather, C. Lee Hanson, said that his son, Peter, was calling from the phone. “As we were talking he said, very softly, ‘Oh my God, oh my God, oh my God!'” The 73-year-old Hanson was describing the moment he watched the plane become the second to hit the Twin Towers.

Mr. Hanson said his son told him he thought the hijackers were going to crash the plane into a building, and his son told him, “Don’t worry Dad, if it happens, it will be quick.”

Sue and Peter Hanson were on their way from Boston to Los Angeles to visit the grandparents and take their baby daughter, Christine, to Disneyland.

The grandfather said that medical examiners asked him in the days after the attacks to retrieve DNA samples from the family home to identify remains. The grandfather said it was “probably one of the worst things I ever did in my life. I was picking hair out of hair brushes, putting toothbrushes into bags.”

He said the only remains that were ever found was a bone of his son, a few inches long.

U.S. District Judge Leonie Brinkema’s caution to prosecutors came after complaints from the defense lawyers that a stream of victim-impact testimony last week would be overly prejudicial to the jury that must decide if Moussaoui is to be executed.

Yesterday, prosecutors told the judge they would display fewer family photos and would try to keep testimony from each witness to under 30 minutes.

Judge Brinkema acknowledged that there is no way to avoid emotional testimony in this case, but reminded them that overly prejudicial testimony can be grounds for overturning a death sentence on appeal.

“You may pay a price for that down the road,” she told prosecutors.

Despite her admonition, the testimony remained deeply affecting. Jurors heard a 911 tape from a trapped victim on the 83rd floor of the South Tower, who told the dispatcher, “I’m going to die, aren’t I? Please God, it’s so hot, I’m burning up.”

The first witness yesterday was John Creamer, an assistant principal from Massachusetts, who described the impact of his wife Tara’s death on the couple’s two children.

Mr. Creamer said he sought advice from a child psychologist on informing his then 4-year-old son about his mother’s death.

“How do you tell a child their mom is dead and she isn’t coming back?” Mr. Creamer said, fighting back tears.

Wen Shi, who lost her husband on September 11, said the couple’s youngest son Richard was only two years old in September 2001, but remembers his father well.

“I told him daddy was in paradise,” she said. She then described how her son took a plane trip to China and looked out the window the whole time to see his father in the heavens.

Moussaoui was watching the witnesses closely throughout the morning. When court recessed, he said in a loud voice “Burn in the USA.” At the lunch recess, he said, “Hollywood deadly circus.”

Prosecutors have said that about 45 such witnesses will testify, chosen from hundreds who wished to testify about the nearly 3,000 deaths that day.

The jury last week heard about a half dozen painful accounts of the human toll exacted in the airborne attacks. They included a New York City firefighter whose friend and mentor was killed when he was struck by the body of a person who jumped from one of World Trade Center towers and the suicide note of a woman who lost her husband when his plane crashed into the towers.

Moussaoui is the only person charged in this country in connection with the September 11 attacks. The jury deciding his sentencing fate has already declared him eligible for the death penalty by determining that his actions caused at least one death on September 11.