The officials in the US city named the victims as Jessica Lewis, 35, and her boyfriend, Austin Wenner, 27. A homicide investigation has begun.
The two were shot dead, and their bodies then put in bags and dumped.
The teenagers spotted a suitcase containing one of the bags on a beach in west Seattle and alerted police.
The teenagers' footage was recorded on 19 June and posted the following day on Tiktok, a video-sharing platform.
In the video, one girl from the group is seen opening the black suitcase with a stick and then complaining of a terrible smell.