Assistant District Attorney Karyn Sinunu-Towery said that Brooks will not face prosecution after further investigation into allegations that Brooks repeatedly hit a teammate on the head with a beer bottle in a June incident.
The district attorney's office said early this month there was no arrest warrant for Brooks after one initially had been filed erroneously.
According to police, Brooks hit teammate Lamar Divens with the bottle three times and then punched him in the face during an early morning argument June 8. The alleged attack left Divens with a 3-inch cut on his forehead, according to police statements.
Police statements said Brooks, Divens and others had been drinking and went to a bar before returning to Brooks' house. Divens pretended to throw Brooks' keys in the grass, though he had actually held onto the keys, prompting the attack. Investigators were told Brooks was intoxicated and had threatened to get a gun.
A memo released Thursday from the district attorney's office in announcing the dismissal of the case states that Brooks and Divens had altercations at Brooks' home in both May and June.
"I have reviewed the memo and agree with its conclusion," District Attorney Jeff Rosen said in a statement. "Cases are not always what they first appear. The value to a thorough and deliberate investigation is a more complete picture of the facts. We cannot prosecute this case because we cannot prove the crime beyond a reasonable doubt to 12 jurors. To file criminal charges when we do not believe we will obtain a conviction is wrong."
The 29-year-old Brooks, a seven-year NFL veteran, started every regular-season game last season for the NFC champion Niners, who lost to Baltimore 34-31 in the Super Bowl. Brooks had 46 tackles, 6.5 sacks, an interception and two forced fumbles in his fifth season with San Francisco.
He was a third-round pick by Cincinnati out of Virginia in the 2006 supplemental draft.