An idiom for "a hidden clue that will unravel a mystery"

I'm looking for a good idiom to express the idea of a clue or a key which if found can unravel or solve a mystery or greater problem. A lynchpin to a mystery or diagnosis with implications of it being hidden or obscured

Solution 1:

Here are some options with the idiomatic expressions in bold:

Once we found out that John Doe's fingerprints were on the gun, we knew we had the last piece to the puzzle. Today, he was found guilty.

John Doe's fingerprints were on the gun, and that did the trick. Today, he was found guilty.

John Doe's fingerprints were on the gun, and that sealed the deal. Today, he was found guilty.

John Doe's fingerprints were on the gun, and that unraveled the knot. Today, he was found guilty.

This one is clunky with the gun example I've been using, so I'll change it to knife.

John Doe's fingerprints were on the knife, and that was our smoking gun. Today, he was found guilty.

Solution 2:

You're probably looking for a missing link (excuse the pun)!

M-W:

missing link noun

something that you do not have and that you need to complete a series or to solve a problem

Police are hopeful that the new evidence will provide the missing links needed to solve the crime.

scientists searching for the missing link