The murderer absconds with a smirk, knowing he has covered his tracks like a pro - no witnesses, no finger- or foot-prints, no stray hairs to betray his identity.
But if a mosquito bit our cocky assassin at the crime scene, it could one day lead to a conviction, according to study published Monday in the journal PLOS ONE.
That is because researchers at Nagoya University have shown that human blood extracted from a buzzing bloodsucker can be traced back to its original owner up to 48 hours after a skeeter has siphoned it off.
"This technique can help police work out who was at a crime scene," lead scientist Toshimichi Yamamoto said in a statement.
"In the future, it might provide evidence that can be used to convict offenders."
No one knew how long human blood drawn by a mozzie kept an identifiable DNA profile, so Yamamoto and a team of forensic scientists decided to find out.
They recovered blood from mosquitos who had bitten volunteers, and then used a technique called polymerase chain reaction - PCR for short - to examine it.
PCR is a standard tool in forensics for amplifying a tiny DNA fragment up to thousands of times.
The researchers found that they could accurately match the minuscule blood traces to the volunteers who had offered themselves up as a meal, even after two days of digestion in a mosquito's stomach.
After three days, however, the blood completely broke down.
The experiments were performed with two species, Culex pipiens pallens and Aedes albopictus, both found throughout much of the tropical and sub-tropical world.
"We hope this will help crime scene investigators collect reliable evidence," Yamamoto said.
With further research, he added, it might be possible to accurately estimate when a mosquito plunged his syringe into its victim.
Most mosquitos do not travel beyond a radius of a few hundred metres and, depending on the species, have lifespans ranging from a few days to a couple months. Females - the ones that bite - generally live far longer than males.