ALBANY, N.Y. >> William Spengler raised no alarms in prison for 17 years and for more than a decade afterward. Well-spoken, well-behaved and intelligent, his demeanor was praised by four straight parole boards that nevertheless denied him parole, worried that bludgeoning his 92-year-old grandmother with a hammer showed a violent streak that could explode again.

After his sentence was up in 1996, he stayed out of trouble until 2010, police said Friday. That’s when Spengler went to a sporting goods store with a neighbor’s daughter, picked out a Bushmaster semiautomatic rifle and a shotgun and had her buy the guns that the convicted felon couldn’t legally possess. On Monday, he used the weapons to ambush firefighters lured to a blaze he set at his house in upstate Webster, killing two people and wounding three others before killing himself.

On Friday, state and federal authorities charged the woman who bought the guns, 24-year-old Dawn Nguyen, with lying on a form that said she would be the owner of the guns she bought for Spengler.

The charges involve the semiautomatic rifle and the 12-gauge shotgun that Spengler had with him Monday when volunteer firefighters Michael Chiapperini and Tomasz Kaczowka were gunned down. Three other people, including two other firefighters, were wounded before the 62-year-old Spengler killed himself. He also had a .38-caliber revolver, but Nguyen is not connected to that gun, police said.

Investigators were still working Friday to confirm their belief that a body found in Spengler’s burned home was that of the sister he lived with, Cheryl Spengler, 67.

U.S. Attorney William Hochul said Nguyen bought the two guns on June 6, 2010, on behalf of Spengler. Police used the serial numbers on the guns to trace them to Nguyen.

“She told the seller of these guns, Gander Mountain in Henrietta, N.Y., that she was to be the true owner and buyer of the guns instead of William Spengler,” Hochul said. “It is absolutely against federal law to provide any materially false information related to the acquisition of firearms.”

During an interview late on Christmas Eve, she told police she had bought the guns for personal protection and that they were stolen from her vehicle, though she never reported the guns stolen. The day after the shootings, Nguyen texted an off-duty Monroe County Sheriff’s deputy with references to the killings. She later called the deputy and admitted she bought the guns for Spengler, police said Friday.

That information was consistent with a suicide note found near Spengler’s body after he killed himself. The rambling, typed letter spelled out Spengler’s intention to destroy his neighborhood and “do what I like doing best, killing people.”

Nguyen is scheduled to return to court on Jan. 8. She declined comment Friday, and a working phone number for her lawyer could not be found.

The .223-caliber Bushmaster rifle, which had a combat-style flash suppressor, is similar to the one used by the gunman who massacred 20 children and six women in a Newtown, Conn., elementary school earlier this month.

As police announced the charges against Nguyen, a clearer portrait of Spengler began to emerge, in the words of wary parole commissioners who kept him locked up until the law said they had to let him go.

At his final parole hearing in 1995, the then-45-year-old Spengler repeated his desire to get out of prison while he still had time to rebuild his life. He also took issue with a previous decision not to release him because the board believed he remained a danger to society.

“You know, the only area of confusion, the last Board, they said that I might be a danger to the community at that time,” he said. “I can’t figure out where in my record it shows that.”

“Well, 13 shots to the head. The grandmother. You killed a 92-year-old woman. We are worried about that,” a board member replied. “There might be another occasion where you lose your temper and you might repeat that behavior. That is what frightens us. That frightens us.”

During four hearings between 1989 and 1995, Spengler quarreled with parole board members over details of his grandmother’s killing, insisting each time he’d only hit her three times on the head with a hammer while evidence pointed to 13 blows, and initially saying he couldn’t explain why the attack happened.

He told the commissioners he took care of his father’s mother in her home next to his because others in the family had difficulty dealing with her, in part because she could be violent. He denied insinuations he was taking financial advantage of Rose Spengler.

The transcripts reveal a well-spoken man, proud to be staying out of trouble in prison and earning positions of trust and responsibility, even time out of prison with a work crew that did renovation work in places including a century-old chapel. The board members mention Spengler testing high for intelligence and noted he came to prison with no other crimes on his record, had only dabbled in drug use and had a spotty work history, mostly as a house painter.

On the day of the killing, he said, he planned to nail shut a basement door to prevent his grandmother from going down and endangering herself. But he said she attacked him, inadvertently kneed him in the groin, and he hit her with the hammer.

“So why do you think you killed her?” Spengler was asked in 1989.

“I still haven’t figured that out. It was matter of just wanting to get out. She was between me and the door,” he replied.

“She was just a little, bitty old lady,” a board member commented.

“I realize that. That’s why I still can’t explain it,” Spengler said.