10:20 AM PT -- Detroit Police tell TMZ Sports the second man involved in the shooting has tragically passed away.

A Lions fan lost his life on Sunday -- after he was shot and killed during a party after Detroit's home game against the Buccaneers.

Detroit Police say the man -- who was in his 40s -- had been at a tailgate at Eastern Market in downtown in the hours after the Lions lost a heartbreaker to Tampa Bay ... when he got into an altercation with another man.

Cops say witnesses told them the two guys squared up as if they were going to fight -- when, suddenly, the man's opponent pulled out a gun and fired.

The guy was struck by bullets and died at the scene. Another person in the area was hit by the gunfire as well -- and is currently in the hospital in critical condition, cops said.

Police told media members the suspect shooter -- who's in his 30s -- is currently in custody.

A further investigation into the matter remains ongoing.

Police Chief James White said the whole situation was "ridiculous" ... adding, "The biggest story that should have been told is the Lions lost and some people had a fight."

"But, unfortunately, we've got somebody who lost their life."