This happened in early May, but is just being reported now ... this after Vegas Metro released a bunch of footage related to the case. The family -- who haven't been ID'd -- called 911 on May 1 to report seeing 2 massive beings, standing at least 8-ft tall, roaming their property.

They say the ETs made their way into their backyard after what they described as a loud crash nearby ... part of which, it seems, might've actually been captured on bodycam footage taken by one of the responding officers that night. As you can see, it's a bright green glowing object that's descending from the sky ... and it was inadvertently recorded.

It appears another unit came out to further interview the family after the cop reported seeing the object -- giving credence, at least at first, to their story ... relayed via dispatch.

Unfortunately, that seems to be the only somewhat solid thing was filmed that night. Despite the family's claims -- they don't have any smoking gun picture or video of the "aliens" they described ... although there is 1 clip taken by them that's being picked apart.

The video shows the family reacting to something past their backyard gate and (possibly in their driveway). Some claim to see two aliens here, but it's eye of the beholder per usual.

In the end, Las Vegas Metro ended up closing the case as "unfounded" after not being able to find anything. Still, the interest it's generated has been huge -- with many believing this is one of the best accounts of an alien encounter possibly ever recorded in modern times.

Of course, it's interesting this is happening now ... on the heels of an alleged whistleblower going to the media to claim there's a secret UFO retrieval program that the US has been a part of for years -- with claims of research and attempted reverse engineering on our part.