After breaking a window to the clinic with the hammer, Grady allegedly poured the gasoline into a room and torched it with a lighter. The flames spread, but they also managed to catch the sleeve of his jacket on fire.
The same records show the blaze quickly sputtered out after Grady drove away in his white van.
A broken window and a small amount of damage were all that showed of the attack by the time authorities got there. But security cameras captured grainy images of the whole thing.
The picture of Grady that was painted this week by federal officials in Green Bay, Wis., was one of a bumbling ideologue driven by religious zeal, not a master criminal with an elaborate plan.
Early reports of the attack said the suspect used some sort of chemical-based bomb. But an affidavit written by FBI special agent Patrick Lynch made it clear that the attack was a crude and mostly unsuccessful attempt at arson.
Grady was charged this week with two federal crimes, one count of arson of a building used in interstate commerce and one count of intentionally damaging a facility that provides reproductive health services.
The documents also provide a clearer picture of how investigators with the FBI, ATF and Grand Chute, Wis., Police Department figured out that Grady was their suspect.
A short time after the fire, according to the documents, he was arrested on suspicion of drunken driving following a collision in the neighboring town of Kaukauna. He was later held for violating the terms of his probation. Investigators soon discovered that the van that was towed in the collision matched the one that showed up in the security camera footage.
After investigators identified him on Monday night as the suspect in the attack, the records show, Grady quickly admitted to it. He said he "lit up the clinic" and also told investigators why.
"Grady advised he has protested against abortions in front of that Planned Parenthood clinic on prior occassions (sic)," the document said. "Grady said he is pro-life, believes in God, and disapproves of the activites (sic) taking place at the clinic."
Those same motives were on display Wednesday at his initial appearance in federal court in Green Bay. Outside the courthouse, a reporter asked him why he did it.
"Because they're killing babies there," Grady replied.
Inside the courtroom, according to the Green Bay Press-Gazette, he interrupted the judge and asked, "Do you even care at all about the 1,000 babies that died screaming?"
He told the judge he was willing to plead guilty on the spot, but the judge set another hearing in the case for April 19.
Read the FBI agent's affidavit: