Short version - It's Eric. He dies off screen.
Long version:
Contact # 3-19
Eric
Location: Unnamed channel, Leadworks, Fiorina 161 Work Correctional Facility
Date: August 11 2179
Contact with: Adult Alien
As this piston starts Ripley ushers Eric into a corridor on one side of the Leadworks while she heads to the opposite side. Shortly afterwards Ripley, her chest pains starting to come more frequently, stumbles across Eric's body. Though he is dead, his death has been left off screen. He is bloodied around the neck, so probably suffered an attack similar to Kevin's.
There are a couple of odd things about this death. One, as mentioned, Eric was on the other side of the leadworks, so how was Ripley able to stumble across his body? One possible explanation is Ripley ended up on the other side, after running down a number of channels and sealing the doors. How she got there so fast is anyone's guess. Another possible explanation is the Alien grabbed Eric and took him over to the other side of the leadworks, then dumped him. However the Alien was on the same side of the leadworks as Ripley; not Eric. Thus leaving us with the more plausible answer that Eric did what he needed to do, then ran behind the piston to the other side and was killed there.
The other odd thing about Eric is how Ripley finds him. She rounds a corner, and finds him lying there – or so we're lead to believe. We only see his head and shoulders, but judging from the shot, they are lying about waist height – not on the ground. It's possible Eric's body is lying on a piece of old equipment (barrels like the one's used for the quinitricetyline?).
It's also interesting to note how quickly Eric was killed. Most of the closing stage of the film happen more or less in real time. Dillon says they have "four or five minutes" before the piston locks into the lead mold. In fact it takes four minutes and thirty four seconds. So from Eric disappearing into the tunnel to reappearing dead on the other side of the leadworks takes just 12 seconds. During this time the Alien has dispensed with Kevin, killed Eric and already taken off after Jude again. That's one quick Alien.
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In the Workprint Ripley verbally identifies Eric when she finds his body.
