In every field office of the FBI, there exists an astonishing group of low-profile, high-energy crime fighters who know more about criminal behaviour than criminals know about themselves.

They are the newest, most effective weapon in the US's crime-fighting arsenal. They are specialists who solve crimes from mischief to murder.

All of their cases were once deemed either impossible to solve, or could take years to build. They are the FBI's elite Evidence Response Teams - ERTs.