More than 15 police officers turned up at the flat of Kovan murder suspect Iskandar Rahmat on Saturday afternoon.

They had to knock close to 20 minutes before the people inside the three-room flat in Block 195 Kim Keat Avenue, in Toa Payoh, would open the door for them.

The police spent about four hours inside before leaving with some bags at about 5pm.

Neighbours said the residents of the unit - an elderly Malay couple and a younger Malay couple - kept mostly to themselves.

A resident in his 70s who lives a few doors down said he recognised Iskandar, 34, a policeman from Bedok division, as his neighbour from the pictures reported in the media. "We hardly talk," he added.

Iskandar was arrested in Johor late on Friday night at a Danga Bay eatery. He went on the run on Wednesday night. He is suspected to be behind the killing of businessman Tan Boon Sin, 67, and his son Tan Chee Heong, 42, also a businessman.

The father was found slashed in his Hillside Drive home while the body of the son, who had also been slashed, was dragged by a car driven by the suspect and dislodged 1km away. The car was found abandoned in Eunos.

Meanwhile, Malaysian media reports said the suspect is believed to have ridden a green Vespa to enter Johor through the Woodlands checkpoint at 11.17pm on the day of the killings.

Cameras at the Immigration and Customs Authority checkpoint recorded the suspect riding a scooter and exiting the checkpoint en route to Johor, said a New Straits Times (NST) online report.

NST said Singapore police released Iskandar's profile and, through Interpol, requested that Johor police put the suspect on a "stop list".

A source familiar with the request said all active duty police personnel in Johor were alerted on the all-points bulletin on Thursday, before being briefed about the suspect's particulars and his vehicle, said NST.