The officer said nine civilians had been evacuated from the shopping mall and security forces were checking whether any remained there.
"The perpetrator is still in the basement and it doesn't look as though he has hostages. We are getting close to him," said the officer who did not give his name because he was not authorized to speak to media.
The killings began at around 3 p.m. (0800 GMT) on Saturday when the soldier opened fire in a house before moving to an army camp and then to the mall in Nakhon Ratchasima in northeastern Thailand, posting messages on Facebook as he went.
Police have identified the suspected shooter as 32-year-old Jakrapanth Thomma.
Overnight, one member of the security forces was killed and at least two others wounded in a raid into the Terminal 21 mall to try to stop the gunman.
