In documents obtained by ABC News, police say they were called regarding a loud argument between a man and woman at an apartment on around 11 p.m. on Oct. 12.
Officers said they could hear the argument from outside the apartment.
They say Foster answered the door and told the officers everything was okay and did not want police to enter his home.
A short time later, Ennis came to the door and stepped outside.
Officers say she was crying and told them she was "not okay."
Ennis told officers she was in a dating relationship with Foster and was upset after finding things on his phone and that was the cause of the argument.
Foster was put in handcuffs while officers continued their interview with Ennis, but the report said she claimed nothing physical happened.
Officers took photos of both Ennis and Foster to document there were no visible injuries to either person.
Santa Clara police say the NFL has reached out to them to get more information on this case.
Calls to 911 of the October incident were also released to ABC News on Friday. Here's a transcript of one of the calls:
911 CALL #1
Caller: I'm a neighbor and I'm hearing loud, you know, someone being banged to the wall or something. You know, lots of...a girl is crying there. I don't know what's happening.
Caller: Some male is shouting, yelling at the peak of his voice, so...
Dispatcher: How long has it been going on?
Caller: Today, I am noticing it second or third time. The last time, I called the apartment security numbers, but they didn't pick up. So today is happening a second time I'm hearing it.
Dispatcher: Right, but I'm just talking about today. Have you been hearing it for the last five minutes, the last hour?
Caller: I think the last 10, 15 minutes I've been hearing this.
The 49ers said they were not aware of the incident until after Foster was cut from the team in November, following another domestic violence incident at a team hotel in Tampa, Florida.
In that incident, Tampa police arrested Foster after receiving a complaint from Ennis, who is now his ex-girlfriend.
Ennis also accused Foster of assault in February 2018, but later recanted, saying in an exclusive ABC News interview that she lied to extort money from him.
Foster pleaded no contest to a misdemeanor weapons charge from that incident.
The Santa Clara County District Attorney's office told ABC7 News after that arrest, it was exploring possible legal options against Foster.
According to our media partner, the San Jose Mercury News, that may include considering re-charging him on the original domestic violence case.
While Foster was released by the 49ers following the Tampa arrest, he was claimed by the Washington Redskins and subsequently placed on the commissioner's exempt list, meaning he cannot practice or attend games, but can be at the team facility on "a reasonable basis" for workouts, meetings and other permitted non-football activities.