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 today.

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

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.

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 today. “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.”

The improvised explosive device, which police say was made from a pipe and weighed more than 6 pounds, scattered bodies, spattered blood, blasted windows and burned motorbikes to the metal. The explosion went off around 7 p.m. in an upscale area filled with tourists, office workers and shoppers.

No one has claimed responsibility.

Bangkok was rattled again today when another explosive device blew up at a ferry pier often used by tourists. No one was hurt.

“The bomb at Sathorn Pier was also a pipe bomb and it might be related to the (Monday) bombing,” said Prawut, the police spokesman.