Santa Clara County prosecutors are leaning against charging San Francisco 49er Ray McDonald with domestic violence in connection with his widely publicized arrest on suspicion of roughing up his pregnant fiancee, sources familiar with the case said Thursday.

The Santa Clara County District Attorney’s Office is likely to announce the decision soon and issue a report laying out the reasons, the sources said, as it has in other incidents involving the 49ers that weren’t related to domestic violence, including not charging 49er Ahmad Brooks with assault after an alleged scuffle with a teammate. However, the office did prosecute Aldon Smith for drunken driving and possession of illegal assault rifles.

The District Attorney’s Office would not comment on the likely outcome of the McDonald case and denied rumors that a report would be issued Friday. Prosecutors have twice canceled a tentative arraignment hearing for McDonald, a good sign for the player, some legal experts said.

McDonald’s supporters have long insisted that the defensive end is a gentle giant who was merely putting up mild resistance Aug. 31 to ward off a physical attack by his fiancee. In late May, the woman grabbed a gun during an argument the couple had and left the house when McDonald threatened to call police. Officers responded to the call, but no arrests were made.

In both instances, the woman could have been defending herself. Police initially said his pregnant fiancee showed “visible injuries” that some sources described as red marks or bruises.

But legal experts have said the May incident also forced prosecutors to examine the woman’s history of aggression, if any. She has retained her own lawyer in the matter.

The case also has been complicated by the actions of Sgt. Sean Pritchard, a San Jose police officer who was moonlighting with team security. Pritchard had attended a party at McDonald’s house shortly before the incident. Apparently responding to a phone call from McDonald after the incident, he returned to the home in uniform and on duty — before other officers arrived in response to a 911 call.

Another complication is that witnesses who may have seen what happened between the engaged couple left the scene before being interviewed, and investigators have encountered difficulty tracking them all down, sources said. It is unclear whether the witnesses left after Pritchard arrived or before.

Pritchard, a gang-suppression unit supervisor, has been banned from moonlighting for the 49ers pending the results of an internal investigation. San Jose police officials also have barred other officers from taking secondary employment assignments related to the 49ers “until further notice.”

In a snippet of 911 tape on the May incident that KTVU obtained, a police dispatcher can be heard relaying information about the incident, presumably to a squad car, saying “an -ex fired a gun, possibly at a male she’s in a verbal with.”

However, the synopsis San Jose police released to the department specifically stated that the woman, who had left the house before police arrived, had held the gun at her side.

Contact Tracey Kaplan at 408-278-3482. Follow her at Twitter.com/tkaplanreport.