He said that having been briefed on the security system, he personally felt that 'the policies in place are sound and the system is sound'.

But the system has to be followed, he added.

Speaking to reporters at a grassroots event in Yishun, he said: 'You can have the best systems in the world but if some of the people don't follow the procedures, then these sorts of lapses will occur.

'Can you ever put your hand on your heart and say that there will never be any human error?

'I don't think anyone anywhere in the world, relating to a large body of men and women, can say that.

'But bear in mind as I say these things, there are no ifs, there are no buts. It was an error, it was a mistake. There's no running away from that. It should not have happened.'

The security lapses at the Subordinate Courts' detention facilities in Havelock Road were exposed last Wednesday when detainees Jamaluddin Salam, 32, and Salman Abu Samah, 28, escaped from the lock-up area and tried to make a run for it.

They were eventually stopped by the police, but Jamaluddin had managed to make it out of the Subordinate Courts building before being brought down 100m away.

The police revealed that two policemen had breached procedure.

One had opened the cell door to pass water to the suspects, who pounced on and assaulted him.

When they proceeded to press the buzzer to open the exit gate, another policeman unlocked it without first checking the closed-circuit television monitor to see who was behind it.

For their failed escape bid, Jamaluddin faced four more charges on Friday, while Salman had two added to his name.

The Criminal Investigation Department is investigating to see if any officers had been negligent in their duties.

Mr Shanmugam, who is also the Law Minister, said the challenge now is to try and 'totally eliminate, or substantially reduce' the probability of such lapses happening by relooking at the system.

He pointed out that the Subordinate Courts' security system had undergone a thorough review in March and April and that changes had been made.

But he cautioned that security should not be made so tight that ordinary citizens cannot use the courts as thousands of civilians go there daily.

One possible solution in the works: using video-conferencing to convey and present pre-trial cases to the Subordinate Courts.

This would ease the level of human traffic moving through the courts and the resources needed to manage them.

He added that the human traffic flowing through the ageing Subordinate Courts building is 'well in excess' of what it was designed for.

Furthermore, 'some of the courts are designed for security, some of the courts are not'.

Asked if he was concerned if Singaporeans' confidence was shaken as the attempted escape happened just four months after Jemaah Islamiah detainee Mas Selamat Kastari escaped from the Whitley Road Detention Centre, he said: 'It's legitimate for people to ask, 'What is happening?'

'At the same time, it's certainly my duty to point out: Look, let's look at this in context. We have a police force that is the envy of many police forces around the world. We have possibly one of the lowest crime rates in the world.

'Let's just ask ourselves: Do we feel safe, secure, to walk around the city and feel that we can go wherever we like without really being affected by crime? The answer must be a 'yes'.'

chengwee@sph.com.sg

Shanmugam: Use more video calls in Sub Courts

Latest 'prison break' a sign of complacency?