Police Commissioner Charles H. Ramsey launched an Internal Affairs investigation into the incident. In February, Williams convened a grand jury to investigate the alleged assault.
After reviewing "countless hours of evidence, surveillance video, and witness statements," Williams said at a news conference Thursday that the grand jury found that police "had acted responsibly and that no criminal act was committed by any members of the police."
The jurors also found that Manning had "exaggerated" details about the officer's actions, Williams said in a statement.
Manning's attorney, Lewis Small, said that he did not agree with the conclusions of the grand jury's report and that police had unnecessarily stopped his client.
"There's no question in my mind," Small said, "that he was improperly stopped, and he was improperly charged, and he was injured by their conduct."
Manning was charged with resisting arrest, simple assault, and related offenses. All charges but resisting arrest were later dropped. He did not admit guilt, but signed an agreement with the court that allowed for the charge to be eventually dropped.
The 36-member grand jury interviewed 25 witnesses, watched video from seven locations, and reviewed hundreds of medical records to establish a timeline of events leading to Manning's arrest Jan. 7.
The panel wrote that Manning and several teammates from his high school basketball team had left the Broad Street Line's Girard Avenue station just before 2 p.m. when a police van passed them. The officers in the van testified that they attempted to stop and talk to the students, who they believed were signaling them. When they did, they testified, some of the students began to run.
The officers caught up with Manning, who they said struggled and hit one of the officers. They called for backup, and a female officer arrived and attempted to frisk Manning. The female officer testified that she did not touch Manning's genitals and that a male officer completed the search.
None of the search was caught on camera, according to the report, and the grand jury relied on "credible testimony from multiple witnesses" to piece together the incident.
The grand jury noted that there was no Police Department directive prohibiting female officers from conducting searches on male suspects.
The officers did not find any weapons or contraband during the search, according to the report.
The grand jury said it was "understandable" that Manning believed he should not have been stopped by police, as he had not committed any crime. But, the panel concluded, he physically resisted the officer who tried to stop and question him.
The grand jury, in reviewing medical records, found that there was no medical evidence that Manning's testicle had been ruptured or undergone trauma.
Manning went to Children's Hospital of Pennsylvania the day after his arrest. The attending urologist there that day, Gregory Tasian, testified that he saw no signs of trauma during an exploratory operation to check for a testicular rupture.
Williams said that the public had been concerned about the allegations against the officers and that he felt a duty to investigate the incident.
"The findings of this grand jury," Williams said, "are exactly why I cautioned people not to rush to judgment in this case."