Police have taken fingerprints of SMRT staff working during the early hours of May 5 - the day that a train was found vandalised just before it left the Bishan depot to start its service.
The Straits Times understands investigators are looking at all leads, including the possibility that the act of vandalism - the third which SMRT has encountered in four years - was perpetrated by staff or contractors.
However, they are not ruling out outsiders, though there is no sign of trespass. Sources said investigators are reviewing the footage of all video cameras in the vicinity of the defaced train.
Unlike the two previous works of graffiti found on MRT trains, the latest appears somewhat haphazard.
A photo of the defaced train obtained by The Straits Times showed large surfaces of a carriage smeared in red, with smudges of white and green.
The two previous ones were well defined and organised.
The first case was in May 2010, when two vandals cut through the fence of SMRT's Changi depot and spray-painted graffiti on one side of a train.
One of them, Swiss national Oliver Fricker, was given seven months' jail and three strokes of the cane, while his accomplice, Briton Lloyd Dane Alexander, remains at large.
In August 2011, a hole was cut in the fence at the Bishan depot, and the words "Jet Setter's" were spray-painted on one of the trains.
Vandalism carries a maximum punishment of three years in jail or a fine of up to $2,000, and caning of three to eight strokes.
Metros in many major cities face the scourge of vandals, who sometimes damage more than just a train's paintwork.
Australia's The Age newspaper reported that vandals tamper with trains' signalling and braking systems, which can potentially lead to serious accidents.
It reported that an average of 35 trains are vandalised each month in Melbourne alone.