I’ve seen a couple of 1-hour documentaries on TV.
There is no clear answer as to what sank the submarine. She signaled the men on shore after the attack. A fire was lit on shore to guide the men home.
Right now it sounds like they bottomed the boat, fell asleep and never woke up. The balast pumps were x-rayed and showed that the crew were not trying to pump out water.
There is a made-for-tv movie about the submarine. It was produced by the TNT network about 10 or 12 years ago. The writers had the men getting stuck on the bottom when they took on too much water.
Those men were frighteningly brave. That thing was a deathtrap,,,with almost no hope of escape.
I thought they died from lack of oxygen. Didn’t they have oil lamps burning inside? IIRC, didn’t they also find the crewmen still in their uniforms/shoes? Anyway, fascinating.