By Steve Tawa

By Steve Tawa

PHILADELPHIA (CBS) — A nationally renowned arson investigator hired by Daniel Dougherty’s defense team has told jurors at Dougherty’s murder retrial that he could not come up with a cause of the deadly fire in the defendant’s home decades ago.

READ MORE: Eagles Center Jason Kelce To Undergo Routine "Cleanout" Elbow Surgery

He testified it should have been ruled “undetermined.” He also clashed with the prosecutor in a day-long verbal scrap.

Assistant DA Jude Conroy accused forensic scientist John Lentini of “expectation bias” because of his investigative methods, including that a fire is presumed accidental until proven intentional.

Lentini told jurors it creates a “speed bump,” because he’s seen over and over again through more than 40-years of investigating fires that people are “falsely accused of arson.”

READ MORE: South Philly Basketball Camp Hosted By Councilmember Isaiah Thomas Defies Intense Heat

Lentini says the lead investigator in the 1985 Oxford Circle rowhome fire, in which the three and four-year-old sons of defendant Daniel Dougherty died, had “expectation bias” once he decided it was intentionally set in 3 places.

Lentini look incredulously to the jury and said, “that must be magic, it can’t be done.”

He says there was no way at that point Fire Lieutenant John Quinn would’ve stepped back and considered if it was accidental.

MORE NEWS: Raising Cane's Opens 2nd Philadelphia Location Near Temple University Campus

Lentini says it looks to him like one fully involved fire.