What cause this line on the film

[mermaid bicycle]

Helping a friend figure out what went wrong here. This was hand developed with the film on reels in a tank.
We don’t think it’s a camera light leak.

Could it be a processing issue and if so what is it?

 

[MattKing]

Can you use a blank background?
Blank.org is a website that works for me.

 

[runswithsizzers]

Does the line look the same on the first few frames as it does on the last few frames?

What evidence do you have that would rule out the possibility of a light leak in the camera?

 

[julio1fer]

I see no lines but the text lines in the background screen

 

[mshchem]

Is there a continuous dark line running the length to the film?? Yeah it's hard to tell anything against the cluttered background

 

[joho]

It looks like the paper backing ???? was the roll of film exposed to x-rays ???

 

[chuckroast]

That appears to be print through from the paper backing. I have seen defective film do this in the past.