Under cover of darkness, the "terrorists" sneaked into Singapore on speedboats on Monday night and launched multiple attacks.
Armed with rifles, grenades and explosive vests, they hit the Marina Country Club in Punggol first before fanning out across the island. They later struck almost simultaneously at shopping malls in Tampines and Bishan, killing civilians at will.
These attacks set the scene for the simulated terror incident faced by security agencies over the last two days, in the largest counter-terrorism exercise held here so far.
Led by the police, the 18-hour islandwide operation that began on Monday morning involved almost 2,500 officers from the Home Team, 660 Singapore Armed Forces (SAF) soldiers and about 60 community volunteers.
The officers responded to attack scenarios at six different locations. The operation climaxed at malls in Bishan and Tampines, where gunmen fired at will and took hostages before they were subdued early yesterday morning.
At Century Square shopping mall in Tampines, four terrorists walked into the mall just after midnight yesterday and opened fire without warning.
Shoppers who managed to run for cover called the police - the lead agency when a terror strike happens here.
Moments later, the first responders from the police's Ground Response Force (GRF) and Emergency Response Team (ERT) arrived on the scene, and they soon overwhelmed and killed the four gunmen.
The GRF, which comprises patrol units nearby, and ERTs, who are trained in counter-assault skills and equipped with HK-MP5 submachine guns, form the first line of the counter-terrorism response.
Troops from the SAF are called in for support when needed.
At the Junction 8 mall in Bishan, police and SAF officers responded to an unrehearsed "active shooter" scenario.
There, the SAF's Army Deployment Force was called in to form a cordon around the mall to contain the shooters, while its Special Operations Task Force (SOTF) officers entered the mall along with the police's ERTs to take down the attackers.
Meanwhile, in another attack scenario in Tampines, police officers pursued terrorists as they moved from Century Square to the adjacent Tampines Mall.
Along the way, the Special Operations Command's Anti-Swarming Teams - crack troops armed with assault rifles and trained in urban combat - were also activated to deal with the threat.
At the Tampines Mall atrium, these officers encountered a terrorist wearing a suicide vest - but they managed to shoot and kill him before he could detonate it.
Later, in an act of desperation, the remaining terrorists holed up in a Golden Village cinema hall, taking about 35 moviegoers hostage.
This was when the SOTF was called in. After they were given a quick briefing by police officers who had formed a security cordon outside the cinema, they stormed the hall in the darkness.
Armed with night-vision goggles, HK-416 assault rifles and P-90 submachine guns, they fired at the terrorists, killing them.
By then, Singapore Civil Defence Force officers had arrived at Tampines Mall to treat the wounded. Those inside the mall were quickly taken to a first-aid point outside, where a medical support vehicle, or mobile hospital, was waiting to tend to those with serious injuries.
Community responders trained in cardiopulmonary resuscitation and first aid were also activated to help the injured. Accountant Rama Diwakar, 45, helped to bandage the wounded. Speaking to The Straits Times after the exercise, Madam Rama said: "This is really very realistic. We've learnt how to treat patients without losing our cool."
With the attacks foiled, one terrorist later attempted to flee the country but was nabbed at the Woodlands Checkpoint by the Immigration and Checkpoints Authority.
The large-scale exercise was held to raise public awareness of the terror threat, and also test the coordination between different agencies.