Jeffrey Dahmer's father says there were at least a couple opportunities to stop his son's murder spree in its tracks -- which he described in gruesome detail to Dr. Phil.
Lionel Dahmer's interview with the TV host is included in a 3-day Dahmer special Phil and co. are airing this week ... and TMZ got a hold of a couple clips from the sit-down addressing two different times when Lionel felt he should've realized something was amiss with his kid.
First, Lionel tells about the time when Jeffrey was still living with his grandmother in Milwaukee -- a period during which he'd already started to kill people -- and how there was a mysterious wooden box Jeff kept that Lionel suspected had pornography inside.
When he tried opening it up, he says Jeffrey was able to buy himself a day ... and then sneak out a severed human head, which he replaced with porno magazines and the like. Lionel says he wishes he would've pressed the issue the day he wanted it open -- because if he did, he says Jeff told him post-arrest that the jig would've been up, potentially saving lives.
Lionel was also asked about what he thinks set Jeffrey down this dark path, and he discusses how when he was young and going through puberty -- he'd mutilate animals and apparently get a sexual thrill by playing with their innards. In hindsight, the signs were there.
BTW, these moments the real-life Lionel talks about are covered in the Netflix series as well ... but Phil's guests, and the victims' families at-large, aren't happy with how it was portrayed.