Three male teenagers are being investigated by the police for spitting on the buttons in a lift at Rumbia LRT station.
The incident came to light last Thursday after SBS Transit posted a photo on Facebook of the lift buttons covered in spittle, describing it as an "abhorrent act".
It sparked condemnation amid the coronavirus outbreak.
Last Saturday, the police said in a statement that it was investigating three 15-year-olds for an offence of mischief.
The teenagers had been caught on police camera, and their identities were established last Friday through ground enquiries by officers from the Ang Mo Kio Police Division.
Preliminary investigations found that they had spat on the lift buttons last Wednesday at about 6.10pm. SBS Transit, the operator of the Sengkang LRT line, had lodged a police report over the incident.
Those convicted of an offence of mischief under Section 426 of the Penal Code can be jailed up to two years, or fined, or both.
Similar spitting incidents have been reported overseas, with people in a state of heightened alert as the coronavirus can be spread through droplets of saliva or mucus from coughing or sneezing.
The police said that they take a "serious view of such inconsiderate and irresponsible behaviour", especially during the current coronavirus outbreak.
"Everyone needs to play his part to maintain social responsibility, and observe good personal hygiene," said the statement.