British Transport Police officers used forensics to trace a man after he spat a member of railway staff on the London Underground.
Investigators identified Hasib Rahimi through his saliva, he was soon arrested and has since been jailed for 10 weeks.
Rahimi, 19, had entered Northwick Park on Tuesday 7 July 2020 and asked for free travel after claiming he had lost his travel pass.
When he was told no, he angrily dialled the police and handed the phone to the staff member.
The victim told the operator there was no emergency, ended the call and then was spat at by Rahimi.
The case was investigated by BTP’s dedicated London Underground staff assaults team.
They used a sample of Rahmi’s spit to identify and ultimately arrest him.