Experts say it's unusual for a serial killer to target two victims at a time.
Investigators are looking at striking similarities between the murders of two Indiana teens and the unsolved killings of two cousins five years ago in Iowa.
Read: Ex-NFL Star Joins the Hunt for Indiana Girls' Killer: 'Let's Rid the World of This Monster'
Lyric Cook, 10, and Elizabeth Collins, 8, were biking together on a remote trail when they disappeared in July 2012.
Their bodies were found five months later near a lake, 400 miles away from the scene of the Indiana murders of 13-year-old Abigail Williams and Liberty German, 14, who were killed while on a hike on February 13.
The cases have eerie similarities. All four girls were attacked on remote hiking trails. They were murdered on the 13th day of the month, and their bodies were discovered near water.
Collins' father, Drew, says there is another detail in the dates of both cases. His daughter and niece was murdered 7/13/12 and the two Indiana teens were killed 2/13/17 and both dates add up to 32. Some say the number is a symbol of evil.
Drew Collins told Inside Edition: "Someone brought it to my attention, I looked at it and if you look it up, I guess it has a lot of meaning to the occult."
A grainy photo was taken of the suspect approaching the Indiana girls on a rickety bridge. The suspect’s voice was recorded on one of the girls’ cell phone saying, "Down the hill. Down the hill."
Read: Citizen Sleuths Spring Into Action in Indiana Murder Mystery as Reward Reaches $50G
Now, police are desperate to catch the killer before he strikes again.
Watch: Speculation Grows as Hunt Intensifies for Killer of 2 Teen Hikers in Indiana
