BANGKOK — Thai investigators believe a man seen in security video wearing a yellow T-shirt and carrying a backpack set off the bomb at a central Bangkok shrine that killed 20 people and injured more than 100, police said Tuesday.

“The yellow shirt guy is not just the suspect. He is the bomber,” police spokesman Lt. Gen. Prawut Thavornsiri told The Associated Press.

Prawut earlier released several photos of the man, with and without the backpack, on social media. The images were apparently taken from closed-circuit video at the Erawan Shrine on Monday evening before the bomb exploded. He confirmed that the man is suspected in the bombing when contacted by The Associated Press.

A video posted separately on Thai media appeared to show the same man, with youthful shaggy dark hair, sitting on a bench at the crowded shrine, then taking off the backpack and leaving it behind as he walked away.

Prime Minister Prayuth Chan-ocha called Monday’s explosion at a busy intersection “the worst incident that has ever happened in Thailand,” and promised to track down those responsible.

“There have been minor bombs or just noise, but this time they aimed for innocent lives,” Prayuth said Tuesday. “They want to destroy our economy, our tourism.”

Without elaborating about possible perpetrators, the prime minister said, “Today we have seen the closed-circuit footage, we saw some suspects, but it wasn’t clear. We have to find them first.”