A teenager suspected of killing two and wounding five others in a Bangkok shopping mall shooting had modified a handgun that was designed to fire only blanks, police said on Wednesday.

Chaos erupted at the Siam Paragon mall close to peak hours on Tuesday, with hundreds fleeing as gunshots rang out. The dead were two women from China and Myanmar.

The suspected gunman, a 14-year-old, was arrested late on Tuesday. Police said he modified a gun designed to fire blanks, to enable it to use live ammunition.

Police had said the suspect had been receiving psychological treatment and had skipped his medication.

"Usually. we don't get the suspect alive. This case is an important case study for us. We need information from the young man," new national police chief Pol Gen Torsak Sukvimol said in a television interview on Wednesday.

"Initially I spoke to him to calm him down because he was worried and appeared to hear someone speaking to him, he was hearing things, a noise he said told him to shoot,"  Torsak said.

The incident at Thailand's most famous mall could be a blow to the country's tourism.