"After a careful review of the information contained in the investigation, we determined that there was sufficient cause to seek an indictment in this matter," Montgomery said. "However, we were unable to meet the necessary burden of proof required to move forward with formal charges. Absent additional information which could support these or other allegations, we will not pursue this matter further."
The ASU investigation included a taped conversation between Winslow and his alleged victim, in which Winslow reportedly apologized for his actions.
On Thursday, the father of the alleged victim called Winslow "a monster" and said the coach's actions drove his daughter to substance abuse and a suicide attempt in August before she revealed her secret.
But, after meeting with prosecutors in recent weeks, the man, who The Tribune chose not to identify by name because it may identify an alleged sexual assault victim, said his family had become resigned to the possibility that charges might not be filed.
"Winslow is not innocent," he said. "He did this, and the police [recorded] a smoking-gun confession."
In Utah, a 15-year-old cannot consent to any sexual activity with someone seven years older and beyond, or someone in a position of special trust. Under Arizona law, if a case involves a victim 15 or older, prosecutors have to prove a lack of consent. The girl's father said he believes that is why charges were not filed.
"It's a legal technicality," he said. "We're going to continue to push for justice and change [for the law]."
The U. suspended Winslow in late February, after reports of the criminal investigation surfaced. University officials later said they would not renew Winslow's contract, which expired earlier this year.
Meanwhile, a team of investigators assembled by the U. to look into allegations of physical and emotional abuse involving Winslow and several student-athletes continues.
Parents of former swimmers allege the school's athletics department overlooked or improperly investigated allegations of physical and psychological abuse and failed to protect the confidentiality of whistle-blowing athletes.
Parents expressed myriad concerns about the coach even before Winslow was suspended and the university announced it would not renew his contract. The Salt Lake Tribune has obtained letters to the university dating as far back as February 2008.