One of fingerprint expert Philip Herd's most memorable cases, in his 25 years on the job, was not about the fingerprints.
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Police investigators had called Mr Herd to a country Victorian motel where a father had used an iron cord to strangle his young son.
He went into the bathroom and dusted the mirror, searching for the oil and wax residue left behind when someone touches a surface.
There were no prints.
But the dust exposed a message from the father, written in the steamy mirror after a shower the night before. It was a confession that explained why he had killed his son.
"And that only got picked up because I decided to powder the mirror," Mr Herd says.
"That's a memorable job. That epitomises what we do. To get that result is fantastic."
The discovery meant the father was later charged with murder, and not the less-serious manslaughter.
He got 24 years in jail.
Mr Herd has spent three decades in fingerprints, after a stint as a sworn officer with Victoria Police.
He is now based in Canberra, at the Australian Federal Police facility in Majura.
Mr Herd is one of the forensic experts prosecutors call to give evidence at criminal trials. He and the team are involved in some 3000 cases a year.
Fingerprints are the unique marks made by a person's fingers, and toes when they touch a surface. As a discipline, fingerprinting is more than 100 years old but remains as useful as ever.
Mr Herd's expertise comes from a five year program he applied for years ago, and built on in the decades since.
Back then, he says, it wasn't considered "real" policing, because you weren't "out there catching crooks".
He says that the forensic discipline is now very popular, and the team has caught their fair share of people committing crimes.
Mr Herd says the process of matching a print left at the scene to another, on a database for example, is still manual. There is no machine that does the work for him.
And the expert, who has given evidence hundreds of times, says it's still a nerve-wracking experience to go to court.
"Of course, we're sure of our evidence, because we know fingerprints are unique."
But sometimes it's the anticipation of not knowing what questions are coming, he says.
Mr Herd says the fingerprint science is strong, so bias is a big topic in the field at the moment. That is sometimes what defence barristers will challenge in court.
He said the team double-checks each other's work and face annual proficiency tests to maintain their skills.
What Mr Herd can say to a court is whether or not a fingerprint left behind at the scene matches another.
What he can't say is when the prints were left there.
And "we don't necessarily say that the person who left that fingerprint is the criminal," he said.
Figuring out that part is the investigators' job.
But Mr Herd says he and the AFP fingerprinting team are not just in the discipline for "catching crooks".
"I think it's important to make people realise that we're not there for just that purpose, we do a lot of other things that are actually more of a help than a hindrance to people."
Mr Herd says disaster victim identification is one of those instances. When there is a disaster overseas, the Australian government sends Mr Herd and others in the team to identify those who have died.
He says it is a great honour to be sent overseas to help get people's bodies home to their families.
And as a former police officer and forensic expert, does he have a favourite cop show?
Mr Herd laughs before replying that it's the comedy sitcom Brooklyn Nine-Nine, a show about as far from reality as it gets.
