Within just six hours of the bank robbery in Holland Village, the suspect was roaming the streets of Bangkok.
Former police officers told The Straits Times yesterday that they were puzzled as to how that could have happened.
The suspect, a Canadian, had fled Singapore last Thursday with $30,000 in cash after allegedly robbing a Standard Chartered Bank branch in Holland Village at 11.25am that day. He is believed to have first escaped by foot towards the nearby Chip Bee Gardens estate.
The suspect flew from Singapore and arrived in Thailand at about 5pm Thailand time (6pm Singapore time), said Major-General Apichart Suriboonya, who heads Thailand's Interpol unit.
He declined to name the suspect, who was arrested on Sunday at Boxpackers Hostel in the Ratchathewi district of central Bangkok.
That the suspect could leave Singapore without being detected at the checkpoints has stumped Mr Joseph Tan, who served in the police force from 1985 to 1992.
"With today's technology, it's surprising that he could have escaped there. Once the police have established his identity, the protocol would be to alert the Immigration and Checkpoints Authority," said Mr Tan, founder of the Crime Library, a voluntary group that tracks missing people.
Retired police officer Lionel de Souza, who has 27 years of experience, said: "He would have been detained at the immigration counter if his name was on the list. Speed is the essence of success. So the question is whether the police did it quickly enough."
In a statement on their Facebook page, the police said they established the identity of the suspect on the day that the crime was committed, but he had already left Singapore.
Mr de Souza said it was unlikely that the suspect could have used a false identity to leave the country.
He added that the police would have provided the suspect's identity to Interpol, in order for the arrest to have been made on Sunday.
Mr Tan believes that with the arrest, the suspect would be "severely punished" by the authorities here to deter others.
"Although nobody was harmed, robbing a bank is a very serious crime. It is where we keep large sums of money and now it seems there is a loophole," said Mr Tan.
If found guilty of robbery, the suspect would face up to 10 years' jail and at least six strokes of the cane.
The man is currently being detained at an immigration detention centre in Bangkok, said Maj-Gen Apichart yesterday.
Singapore does not have an extradition treaty with Thailand. But the police said the Singapore authorities are "engaging our counterparts in Thailand to see whether the suspect can be released into our custody".