1:54 PM PT -- Sources with direct knowledge tell TMZ ... the shooting Tuesday was completely unrelated to the RNC -- and the reason Secret Service was on the ground was because agents happened to be in the area and simply rendered aid.

A homeless man was reportedly shot and killed not far from the Republican National Convention -- and while it doesn't seem tied to Donald Trump ... cops were out in force.

The shooting took place Tuesday in Milwaukee, about a mile outside the Fiserv Forum -- where there's a wide perimeter of security ... and where a police officer from Columbus, OH opened fire on a man -- who, according to eyewitnesses, was a vagrant in town.

The exact circumstances of the shooting aren't clear -- including why the man was shot -- but all reports indicate the man is dead now ... and there's a massive police presence.

Video of the aftermath of the scene is going around ... and you can see officers swarming the place -- with caution tape up everywhere. The Secret Service is also reportedly on the ground ... although it's unclear what their involvement is in the case.

Of course, you can imagine law enforcement is more on edge than ever in the wake of the assassination attempt on Trump's life over the weekend ... where he was almost killed.

We know his Secret Service detail has been beefed up, and that security at the RNC in general is operating at maximum capacity -- and the fact this many cops were dispatched to deal with this officer-involved shooting, which seems unrelated to Trump, proves that.

In terms of why an Ohio cop is in Milwaukee -- there are actually several law enforcement agencies from neighboring states that have been called in to assist with the week-long event.