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.
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According to a previous report, local public transport operator SMRT lodged a police report on a case of vandalism at its Bishan depot on Monday morning.
In a written statement, the police said that a train was vandalised at Ang Mo Kio Ave 1 on May 5, 2014, at 6.17am.
Police investigations are on-going.
This is the second instance of vandalism at SMRT's Bishan depot.
The first case took place in August 2011, where graffiti was discovered on a northbound train.
In another incident of a MRT train being vandalised, Swiss national Oliver Fricker and his British friend Dane Alexander Lloyd broke into SMRT's Changi depot and vandalised a train in May 2010.