Huff, an FBI agent for 10 years, testified for more than an hour on the first day of Gosnell's murder trial. He is to return to the witness stand when the trial resumes Tuesday.
Gosnell, 72, is charged with seven counts of first-degree murder - newborns whose spines he snipped with scissors after late-term abortions. He faces the death penalty if the jury finds him guilty.
Also on trial is Eileen O'Neill, an unlicensed medical school graduate working as a clinic doctor. O'Neill, 52, of Phoenixville, is not charged with murder but with working in a "corrupt organization."
The Common Pleas Court jury of seven men and five women spent most of the trial's first day hearing opening statements.
Assistant District Attorney Joanne Pescatore told the jury the case is about murder, not abortion.
Pescatore called Gosnell, 72, a "wolf in sheep's clothing" who "used and abused desperate women."
Some, Pescatore said, were untrained workers he persuaded to do medical procedures. Others were poor women desperate to end pregnancies.
Pescatore said Gosnell specialized in what she called "high-volume, high-profit" illegal late-term abortions that "almost guarantee a live birth."
"If a baby is born alive, it's alive, and no one has the right to take some solution to kill it," she added.
Defense attorney Jack McMahon gave the jury a starkly different portrait of Gosnell: a native West Philadelphian with a medical degree from Thomas Jefferson University who decided to stay and serve the neighborhood rather than take much higher-paying offers.
McMahon said prosecutors wanted to impose "Mayo Clinic" standards on a West Philadelphia clinic that served the poor for 31 years: "If you want Mayo Clinic standards, then you go to the Mayo Clinic."
McMahon urged the jury to keep an open mind, saying that until Monday, he and Gosnell had been unable to make public their defense because of the judge's gag order.
McMahon argued that all doctors lose patients but that prosecutors are redefining medical complications as murder: "It fits their needs, this elitist, racist prosecution, to make this a homicide."
The jury has eight black and four white members.
O'Neill's attorney, James Berardinelli, told the jury in his opening that "my client has killed no one."
Berardinelli said O'Neill was supervised by Gosnell in the clinic's family practice and had nothing to do with abortions.
O'Neill was conditionally licensed in Delaware, Berardinelli said, "but they arrested her first."
Questioned by Assistant District Attorney Edward Cameron, Huff testified that the 2010 raid was related to an investigation into reports that Gosnell was selling prescriptions for dangerous narcotics flooding the neighborhood.
Gosnell was later indicted by a federal grand jury in the drug probe and is awaiting a September trial.
But the discovery of evidence that Gosnell was performing illegal late-term abortions took precedence.
Huff said his supervisor arranged for an ambulance for some patients but others required Gosnell to perform the abortion while agents searched.
At one point, Huff continued, Gosnell returned to the clinic office to resume being interviewed and have some dinner.
"He came back and ate his food still wearing the bloody, torn latex gloves," Huff testified.
Gosnell, dressed in a charcoal double-breasted suit, listened placidly to the testimony, hands in his lap, a faint smile playing on his lips.
Only when Huff described the clinic as "unsanitary" and filthy did he frown. Several times he grunted and gestured to show McMahon items in photographs.
In addition to the seven alleged murders of newborns, Gosnell is charged with the third-degree murder of a Virginia woman, Karnamaya Mongar, 41, who was allegedly administered too much anesthesia during a 2009 abortion.
McMahon argued that Mongar, a refugee from Bhutan in the United States four months before her death, did not tell Gosnell or his staff that she had respiratory problems that made her more vulnerable to anesthesia.
Contact Joseph A. Slobodzian at 215-854-2985, email@example.com, or @joeslobo on Twitter.