He attacked his brother Jesse and then threatened to kill everybody in the house, which included his dad Phil and two other women. Then he ran into the woods behind the house before cops arrived.
9:21 AM PT -- TMZ has obtained the police report describing what allegedly happened in the lead-up to Bam Margera fleeing into the woods -- and it's a shocking account.
According to cops -- who say they spoke to the alleged victims, who happen to be Bam's family members in this case -- the former 'Jackass' star started banging on his brother Jesse's bedroom door around 8 AM on Sunday ... and left an ominous note behind.
Jesse told police the signed letter from Bam read, "If you even f***ing think of calling the police on me I will officially f*** you up." Jesse goes on to say he then went downstairs to find Bam peeing into the kitchen sink, at which point ... a fight allegedly broke out.
In the docs, Jesse claims Bam wailed on him and struck him multiple times in the head -- and cops make a note of this in their report, noting visible injuries they say they observed.
After the brawl, Jesse goes on to allege that Bam said, "I'll kill you. I'll put a bullet in your head." Jesse also told police that Bam allegedly said he'd also kill everyone in the house ... the occupants of which, at the time, included his dad Phil and two other women. Jesse told police that Bam fled into the woods behind their house after this ... and cops say they came up short when they went looking for him.