5-year-old killed: How ballistics track down a killer

As investigators search for a motive and a suspect in the shooting of 5-year-old Laylah Petersen, ballistics experts show us how they track down a killer with evidence.

Rich Paigentini, a ballistics expert with Waukesha County Technical College, shows us how officers use a single bullet to track down a suspect.

He says once a victim is shot, evidence is left behind.

Paigentini fires a round and says, "So now the gun ejected the casing, and under the microscope there are going to be marks here that are specific to that gun on how it got ejected."He says if the same gun was used in a different crime, a database search finds the unique pattern on the casing to narrow down a suspect.

Paigentini says, "If the officers recover this evidence and find the person who has this gun and match them up, it starts building a case regardless of what that person is telling them."

He says ballistics experts can also recreate a scene, by using rods to figure out the angle and location where a gun was fired from. They can then figure out which direction the criminals went, and witnesses that may have seen them from that vantage point, all with a single bullet.

Paigentini says, "Each one of these are manufactured by a machine but they're not identical, none of these are identical, it has a unique fingerprint, exactly"

A fingerprint that police are hoping will find the killer, of an innocent little girl.