Bangkok police say a man wearing a yellow t-shirt is definitely responsible for setting off a bomb near a Bangkok shrine Monday, killing 20 people and injuring at least 140 others.

Security footage apparently shows the man—who was wearing a backpack over his shoulders and holding another bag in his hand—sitting down on a bench near the shrine, taking off the backpack, and walking away. He was later seen leaving the area on a motorcycle taxi.

Inside the abandoned backpack, cops say, was a six-pound bomb made of a pipe wrapped in cloth that ripped through a busy intersection in what authorities are calling “the worst incident to ever happen to Thailand.”

“The yellow shirt guy is not just the suspect. He is the bomber,” a Bangkok police spokesperson, Lt. Gen. Prawut Thavornsiri, tells the AP.

Still, authorities say they’re not sure if the man in the yellow shirt was working alone, what his motivations are, or whether he was connected to a second bomb that went off Tuesday after someone discovered it on the Taksin Bridge in Bangkok. The bomb was apparently thrown into a river, where it exploded. This time, there were no reported injuries.

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