The Santa Clara County district attorney's office has declined to file charges against San Francisco 49ers defensive lineman Ray McDonald in a domestic violence investigation stemming from his Aug. 31 arrest.

Prosecutors said in a release Monday that they were unable to charge McDonald because of conflicting versions of what happened, a lack of verifiable eyewitnesses and a significant lack of cooperation by the alleged victim, McDonald's fiancee.

"All domestic violence complaints deserve our concern, sensitivity and careful review," District Attorney Jeff Rosen said in the release. "After our thorough review of all the facts, we do not have evidence sufficient to convince a jury beyond a reasonable doubt that Mr. McDonald committed a crime" against his fiancee.

The DA's statement said no one was left with "significant injuries."

The arrest came only days after NFL Commissioner Roger Goodell announced stiffer penalties for players accused of domestic violence, including a six-week suspension for a first offense and at least a year for a second. That move followed scrutiny over former Baltimore Ravens running back Ray Rice's two-game penalty stemming from his arrest on an assault charge in February.

According to the DA's office, an investigation determined that McDonald's fiancee, who was identified only as "Jane Doe" in the statement, struck McDonald during an argument at his home during his birthday party and he then tried to restrain her.

The two scuffled and McDonald forcibly attempted to remove her from the home, the statement said.

McDonald called a San Jose police officer, saying he needed to get "a female" out of his house, the statement said. The fiancee subsequently called 911 and that led to McDonald's arrest.

According to a memo prosecutors sent to Rosen, both McDonald and his fiancee said she struck him first. The memo says her injuries were "consistent with restraint and an ensuing scuffle, not an attack."

McDonald and his pregnant fiancee got into a fight after a woman attending the party told her some women were getting too friendly with McDonald. McDonald's fiancee, who was sleeping upstairs, came down and confronted him, according to the prosecutors' memo.

She asked a woman talking to him to step outside, according to the memo, and the two got into a fight. McDonald grabbed her arms to restrain her, according to the memo, and at one point grabbed her neck, as he tried to remove her from the house. Both times he left visible injuries, according to the memo.

Following the arrest, 49ers CEO Jed York, general manager Trent Baalke and coach Jim Harbaugh repeatedly said they have zero tolerance for domestic violence, but rather than bench McDonald they agreed to let "due process" play out before making a decision about his future with the team. Harbaugh said he wouldn't allow any player found guilty of domestic violence to be on his team.

He also said at the time that public pressure wouldn't affect how the 49ers handled McDonald's situation.

An eight-year NFL veteran, McDonald has been a key member of one of the NFL's fiercest front seven units.

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