A photo of a singed-but-standing box of Frosted Flakes helped reveal the three-decade-old circumstances of a fatal rowhouse fire in Oxford Circle, a prosecution expert testified Monday.

The condition of the box and its position in the kitchen helped determine the fire's path, consultant and former Philadelphia Fire Marshal Thomas Schneiders said, among factors that proved investigators got it right in 1985 when they said the blaze that killed brothers Daniel Dougherty Jr., 4, and John Dougherty, 3, was deliberately set.

Their father, Daniel Dougherty, was convicted and sentenced to death for murder and arson in 2000. Dougherty, 56, insists that he's innocent, and hopes that advances in fire science will vindicate him.

On the sixth day of Dougherty's retrial in Philadelphia Common Pleas Court, Schneiders said he based his conclusion of arson on old case-file photos, interviews conducted at the time of the fire, and his own visit to the Carver Street home. He fully supported the 1985 findings of Assistant Fire Marshal John Quinn, who said the fire was set in three places: a sofa, a love seat, and under a dining-room table.

"A fire that was intentionally started," Schneiders said. "There was no accidental cause."

The defense contends that Quinn had failed to recognize improvements in fire science and arson detection in the 15 years between the blaze and Dougherty's trial.

The failure of Dougherty's original trial lawyer to challenge Quinn on that point led directly to an appellate court granting a retrial.

Schneiders, who runs TDS Fire Investigations & Consultants in Philadelphia, said the evidence of arson was clear.

Heavy burning was found at the floor and low wall areas near the couch and love seat, but not in other nearby places. The underside of the dining-room table was deeply charred, the top almost unscathed, indicating it was burned from beneath.

Experienced fire investigators, he told the jury, can read burn patterns in a room as readily as a diner in a restaurant can tell whether a hot dog has been boiled or grilled.

Quinn is too ill to testify.

On cross-examination, defense counsel David S. Fryman asked Schneiders whether fire science had improved between the time of the fire and the first trial.

"In some areas," Schneiders replied.

He agreed that, although more than 100 photos were taken at the fire scene, none was shot of the ceiling; and that Quinn never tested a stereo as a potential source of the fire, having already determined that the points of origin lay elsewhere.

Schneiders agreed that the first-floor living room and dining room reached what is called "full-room involvement" - an inferno in which a fire in a room changes to a room on fire.

He read, or had read to him by Fryman, sections from the current National Fire Protection Association guide for scientific investigation and analysis.

It warns that in cases of full-room involvement, damage at the floor level can be extensive, and that "an area of extensive damage may simply mean there was a significant fuel package" - such as a piece of furniture - "at that location."

In this case, the cause of the fire is crucial.

The defense expects to call its own expert to say the cause of the fire should have been classified "undetermined" and may have been accidental.

Dougherty, his live-in girlfriend, and other visitors to the home were smokers.

Dougherty said from the start that he awoke to a house ablaze, ran outside, then tried desperately to rescue his sons. His death sentence was vacated in 2012, becoming a life sentence.

The prosecution is expected to rest on Tuesday, the defense to begin presenting its case.

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@JeffGammage