Thai police have released a sketch of the man whom they say carried out Monday’s bomb attack on a popular shrine in Bangkok that killed 20 people and injured more than 100 others.

Police say the bomber was part of a network, and are offering an equivalent of about $28,000 as reward for information that can lead to his arrest. Police released security footage that show a man wearing a yellow T-shirt and shorts, and carrying a backpack inside the Erawan shrine. Moments later, he removes the backpack, deposits it on a bench and walks off.

Thai Prime Minister Prayuth Chan-ocha said the man was likely hired to plant the bomb, and urged him to surrender.

Police spokesman Prawut Thawornsiri said in a TV interview Wednesday that two other men seen in the CCTV footage—one in red, the other in white—were also suspects in the investigation.

The explosion at the popular tourist attraction killed 20 people, mostly Thai, but also Chinese nationals, who are Thailand’s largest group of foreign tourists.

No one has claimed responsibility for Monday’s attack, nor for the explosion Tuesday at a Bangkok pier. No one was injured in that attack. National police chief Somyot Poompanmoung said Wednesday it was unclear if the two explosions were connected.

Soon after Monday’s blast, Thai officials appeared to blame supporters of the elected Thai government, which was deposed in a military coup last year. But on Wednesday, Poompanmoung was more measured: “I am confident that there are Thais involved but I am not saying it is just Thais or that there are foreigners,” he said at a news conference.

And, he added, the suspects was part of a network. “Not one person can do this,” he said.

The Erawan shrine, sacred to both Hindus and Buddhists, reopened Wednesday. The statue of the god Brahma—the creating deity in Hinduism—suffered only minor damage.

Security has been bolstered in the country following the attack, which the prime minister described as the “worst incident that has ever happened in Thailand.”

Although political unrest is common in Thailand and there have been low-level explosions over the years, the scale of Monday’s attack—and the death toll—are unprecedented. Thai officials downplayed suggestions that Muslim separatists from the country’s south were involved, saying it did not match their methods.