It does question some decisions along the way. For instance, it notes that prosecutors took too long to take certain investigative steps, including gathering reports on Sandusky from other law enforcement agencies, the sources said.
The report was commissioned and will be released by Attorney General Kathleen G. Kane, who was elected in 2012 largely on a campaign that questioned whether one of her predecessors in the job, Gov. Corbett, deliberately slowed the investigation for political purposes.
In her bid for office, Kane said, "Instead of using a grand jury in the Jerry Sandusky case, I would have had him arrested after the first victims came forward."
Kane, a Democrat, suggested that Corbett, a Republican, delayed the probe to avoid angering voters and donors. Politics, she told the editorial board of one newspaper, "probably" drove his decisions.
"You don't put a case like that before the grand jury," she said. "That was the leadership. Somebody made that decision that they're going to drag that out."
However, the review by former federal prosecutor Geoffrey Moulton concludes it was reasonable for prosecutors to build a case with many victims, the sources said. The Moulton report notes that the first victim to come forward had difficulty talking about his abuse, to police or a grand jury, and would often give only one-word replies when questioned about Sandusky's attacks.
A spokesman for Kane on Sunday night said she would have no comment before her news conference Monday.
As for Corbett, Moulton's review of the prosecutors who worked for him - as well as of tens of thousands of e-mails they exchanged during the investigation - turned up no evidence that "electoral politics" played a role in the case. Corbett's critics have suggested he might have delayed such a controversial investigation because he was running for the state's highest office at the time.
The sources said Moulton also concluded neither Corbett nor any of his senior aides told prosecutors how to conduct the investigation.
The Moulton study does question aspects of the case, according to the sources. It identifies several months in 2010 when the inquiry appeared to have ground to a halt.
Moulton also faulted prosecutors for waiting too long to ask Penn State to turn over any records containing complaints against Sandusky. The key subpoena to the university was not issued until December 2010 - 21 months after the attorney general obtained the case. State investigators also did not visit local and university police to search for Sandusky-related records until January 2011, a step that turned up four more victims.
Moulton reportedly also faulted prosecutors for waiting until June 2011 to search Sandusky's house. In rebuttal, the sources said, prosecutors said that for months, they lacked evidence for a search warrant. There was also worry that a search, if publicized, would reveal the open investigation.
The search of the house turned up photographs of Sandusky's victims, as well as a typed list of children's names - some with asterisks next to them. One of those highlighted was a victim prosecutors did not know about at the time.
Sandusky, for years a top assistant to Joe Paterno, is serving 30 to 60 years in prison after being convicted of sexually assaulting 10 boys. Former Penn State president Graham B. Spanier and two other former ranking administrators await trial on accusations that they covered up Sandusky's conduct.
In a timeline Moulton explored in his report, the Sandusky case began in 2008, when a teenage boy told school officials in Clinton County that Sandusky had molested him. Sandusky was not charged until November 2011.
In 2008, social workers found the complaint credible, but the case soon got bogged down in jurisdictional issues. Though the victim attended school in Clinton County, most of the wrongdoing took place in neighboring Centre County, home of Penn State.
Finally, five months after the victim first spoke out, the case was handed off to state prosecutors.
In a missed opportunity, the report says, according to one source, social workers early on conducted what turned out to be the only official interview of Sandusky - but without police or prosecutors.
Once the Attorney General's Office took up the case in March 2009, the assigned prosecutor, Jonelle Eshbach, and her supervisor, Deputy Attorney General Frank G. Fina, agreed to use a grand jury to investigate further. Eshbach believed that the first victim's account needed corroboration and that there were more victims to be found, the sources said she told Moulton.
Fina stressed the secret nature of a grand jury investigation, saying it would give fearful witnesses and victims protection to testify confidentially against such a prominent community figure as Sandusky.
Once the grand jury turned up evidence to buttress the first victim's account, Eshbach wanted to arrest Sandusky, the sources said Moulton learned.
However, the report says, proceeding with an arrest had its own risks – notably, that an acquittal as the result of a weak and thin case would have put a halt to more victims surfacing. And though sex criminals are thought to be obsessive, Sandusky was aware he was under law enforcement scrutiny, the report noted.
The sources said the report takes pains to account for the perspectives of the prosecutors who brought the case and those who criticized it, and recommends ways to improve the process.
The report will be closely read in political and legal circles. A scathing report that faulted Corbett, who faces a difficult reelection battle in the fall, could have damaged his chances for a second term.