Over the July 4 weekend in 1966, three friends from Illinios left for a day at the beach. They were seen buying magazines and sunscreen, and seen lounging on the beach until around noon, when another couple noticed the girls enter the water and get on a boat. They were never seen again.

Sources:

What is the Likeliest Solution to the Indiana Dunes Mystery? | by William Keckler | Medium

YOUNG WOMEN LOST — American Hauntings (americanhauntingsink.com)

Richard Speck - Wikipedia