National police chief General Wichean Potephosree on Wednesday attributed about 10 kilograms of TNT explosives to have caused the Bang Bua Thong explosion last evening.
"From forensic checks, investigators suspect the bomb making was in progress before the explosion," he said.
Based on the explosives found, investigators were pursuing leads that the type of bombs being assembled might be linked to four previous bomb attacks in the capital, he said.
He said there was no evidence to link Saman Boonprasert, the owner of the apartment building where the explosion happened, to any terrorist activities.
But the room was rented by Samai Wongsuwan, a Chiang Mai man, who is known as a red shirt remaining at large with an arrest warrant for involving in a bomb attack in Mae Ping district of the northern province, he added.
The search of the scene found an assault rifle. And the death toll increased to five people.