The 54-year-old actor - who has been accused of domestic violence against Oksana Grigorieva, the mother of his eight-month-old daughter Lucia - allegedly pulled a gun on the Russian singer and said he would shoot her, their daughter and her 12-year-old son Alexander, before turning the weapon on himself.
Speaking about the January 6 incident, a source told RadarOnline.com: "Mel punched her in the mouth and then again in the side of the head.
"Oksana has told authorities it was horrific and she thought she was going to die."
The insider claims Oksana, 40, has told authorities she was holding Lucia during the alleged attack and fell backwards on the bed as the actor tried to choke her.
The `Braveheart` actor then reportedly yelled "f**k you" and called Oksana a "f**king ****".
Alexander - whose father is former James Bond star Timothy Dalton - is believed to have witnessed the row in the bedroom and was interviewed by child protection services agents on 16.07.10.
Mel is now facing a criminal investigation on domestic battery charges by the Los Angeles County Sheriff`s Department and is part of a Department of Children and Family Services investigation.
Investigators will also be looking at the actor`s phone records as it was claimed that after Oksana fled with the children - barefoot and in her pyjamas - Mel repeatedly called her and threatened her.
Meanwhile, Oksana is reportedly planning on filing a suit for defamation and slander against Mel over the expletive-laced, hate-filled recordings that were recently leaked online.
A source explained: "Oksana`s reputation and character has been damaged after what Mel said. He has made her out to be a gold-digger and extortionist, which she is not, and she plans to defend herself against his baseless claims."