The grandfather of the youngest victim of the Sept. 11, 2001, attacks, testifying at Zacarias Moussaoui's sentencing trial Monday, described watching on television as the plane carrying his son and granddaughter hit the World Trade Center.

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 before he watched the plane become the second to hit the Twin Towers on that fateful day.

A few minutes before, Hanson said, Peter had 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 then 2-1/2-year-old girl, 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.

The testimony came as a federal judge warned prosecutors against relying too heavily on emotional eyewitness accounts from Sept. 11 families to influence the jury in Moussaoui's trial.

The word of caution by U.S. District Judge Leonie Brinkema 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.

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

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, its so hot, I'm burning up."

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

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 child their mom is dead and she isn't coming back?" Creamer said, fighting back tears.

Moussaoui was watching the witnesses closely throughout the morning. When court recessed, he said in a loud voice "Burn in the USA."

Prosecutors have said that about 45 such witnesses will testify — compared, they noted — with the nearly 3,000 people who died that day.

The jury already has 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 to avoid being burned alive 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 Sept. 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 Sept. 11.

Even though he was in jail in Minnesota at the time of the attacks, the jury ruled that lies told by Moussaoui to federal agents a month before the attacks kept them from identifying and stopping some of the hijackers.

Now they must decide whether Moussaoui deserves execution or life in prison.

Defense lawyers hope the jury will spare Moussaoui's life because of his limited role in the attacks, evidence of mental illness and because they say his execution would only fuel his dream of martyrdom.