More than 4kg of heroin was found in a toilet bowl and other spots in a Bukit Batok Housing Board flat during a Central Narcotics Bureau (CNB) operation on Monday.

The drug bust netted a total of 5,130g of heroin, 1,125g of methamphetamine also known as Ice, 13g of new psychoactive substances and 20 Erimin-5 tablets, CNB said yesterday.

In addition to the seized drugs, estimated to be worth almost $700,000, CNB officers also recovered $174,000 in cash.

CNB officers intercepted a vehicle in Sunset Way on Monday afternoon.

They arrested two Singaporeans, a 44-year-old man and a 37-year-old woman, and seized 10 packets containing 80g of heroin after searching their vehicle.

A separate CNB team raided an HDB flat in Bukit Batok Street 31, where they believed they would find a 63-year-old Singaporean man who had met the 44-year-old earlier.

Before the CNB officers could enter the flat, the 63-year-old man climbed out of the kitchen window and fell.

He was pronounced dead by Singapore Civil Defence Force officers when they arrived at the scene.

Two more people - a 65-year-old Singaporean man and a 33-year-old woman, a foreign national, - were nabbed in the same Bukit Batok flat.

Drugs including 4,790g of heroin were recovered from various parts of the flat by CNB officers who also found the $174,000 cash haul.

On Monday evening, CNB officers also raided a shophouse unit in Tanjong Pagar Road.

Two Singaporean men, aged 62 and 64, were arrested and CNB officers also seized 35 packets containing 260g of heroin, and 1g of Ice from the shophouse.

CNB said that investigations are under way, adding that the police are looking into the 63-year-old man's death.