Police say they have now secured the Terminal 21 mall in the northwestern city of Nakhon Ratchasima – also known as Korat, but they have not said whether the gunman, named as Sgt Jakrapanth Thomma, has been found or not.
There are no more bodies left inside, according to Public Health Minister Anutin Charnvirakul.
He confirmed the figure of 20 dead, but added: “We don’t know whether there are any additional injuries or deaths or not.”
Just after 2.40 am local time (7.40pm GMT), a large group of armed police officers entered the mall and several bursts of automatic gunfire could be heard from inside.
Mr Anutin earlier said that a doctor was shot while helping an injured victim.
The suspect appeared to be armed with an assault rifle, based on security camera video aired on Thai Rath television.
Several Thai media reported that he travelled in a military vehicle.
Earlier, defence military spokesperson Kongcheep Tantrawanit said: “Police officers and soldiers are joining forces and have helped evacuate hundreds of people from inside the mall. It’s not known how many are still inside.”
Thai gunman walking through mall
Dozens of people were seen running from the building, fleeing in groups.
Some carried children, while others supported elderly people, carrying them with one person on each side.
Authorities say they got reports of the shooting at around 3.30pm local time. Police announced they had secured the mall shortly before midnight.
The gunman initially went to a house in the city, which is more than 250km (155miles) from Bangkok, and shot two people dead before going to a weapons store on an army base and taking a new gun, local police said.
Earlier in the day he posted on his Facebook page that: “Death is inevitable for everyone.”
At one point after the shooting had begun, he wrote: “Should I give up?”
He also uploaded a picture of what looked like him holding a gun, and in a later post, he wrote: “I have stopped already.”
Facebook said on Saturday that it had removed the account, saying: “Our hearts go out to the victims, their families and the community affected by this tragedy in Thailand.
“There is no place on Facebook for people who commit this kind of atrocity, nor do we allow people to praise or support this attack,” a representative said in a statement.
The soldier opened fire at different locations in the city, police spokesman Kissana Phathanacharoen said in a message to reporters.
He then drove past a school towards the Terminal 21 shopping centre and continued his shooting spree.
It was shut down and the street outside closed.
Video taken outside the shopping centre and shared on social media show people taking cover in a car park as gunshots were fired.
A woman interviewed by Thailand’s Channel One television said when she heard gunshots she hid in a clothing store with other people before managing to escape.
Another woman who filmed a video from outside the shopping centre said she had just left the building after finishing her meal.
“While we were in our car on the opposite side, we heard a loud sound and then we could see people starting to run out”, said Nattaya Nganiem.
She added: “First I saw a woman run out from the mall hysterically. Then a motorcycle rider in front of her just ran and left his motorcycle there.
“I thought, what’s happening? Then we heard several gunshots.
“The motorcycle guy probably had seen the gunman from where he was. We couldn’t see him, but we could hear the sound of the gun. It was terrifying.
“We were just inside having dinner and my child was getting unusually restless, that’s why we left, otherwise, we would have been stuck inside there.
“I can’t believe this is happening in my hometown. I mean, this shopping mall, we go there almost every other day.”
She said once she got home and learned what had happened, she fainted.
A police officer said the suspect had been involved in an argument about a land deal and shot two people who were involved, another soldier and a woman.
He said a third person was injured.
“We don’t know why he did this. It appears he went mad,” said defence ministry spokesman Kongcheep Tantrawanit.
Prime Minister Prayuth Chan-ocha has expressed condolences to the families of those killed.
