Police in Thailand released a sketch on Wednesday of the man they believe carried out this week's deadly Bangkok bombing, and offered a 1 million baht ($28,000) reward for help leading to his arrest. But apart from a rough portrait, authorities have few solid leads they don't know the bomber's motive, where he's from or if he's still in the country.

The sketch was released after grainy security video footage showed the man leaving behind a backpack just 15 minutes before the blast at a popular downtown shrine. Police said the picture was also partly based on a description provided by a motorcycle taxi driver believed to have given him a ride on the night of the Monday blast. Two days after the attack, which authorities have called the worst in Thai history, the open-air Erawan Shrine reopened to the public. But little is known about who carried out the blast that left 20 people dead and more than 120 injured, and no one has claimed responsibility.

Police chief Somyot Poompanmoung said authorities didn't know if the suspect seen in the video was still in the country, whether he was Thai or a foreigner and were wondering if he wore a disguise to mask his identity. "If citizens or anyone can give us information or clues that leads to the arrest of this man I have set a reward of 1 million baht," Somyot told reporters, adding that police believe the bomber worked with accomplices. "He didn't do it alone, for sure. It's a network," Somyot said. Police spokesman Lt. Gen. Prawut Thavornsiri said the security footage appears to show two possible accomplices who are also considered suspects.

On Tuesday, police released still photos of the man seen in the video, wearing a yellow T-shirt and shorts. His hair appears shaggier in the video than the police sketch. The photographs show him both with and without a large, black backpack. A video posted separately on Thai media showed the same man sitting on a bench at the shrine, taking off the backpack and leaving it behind as he stood up to fiddle with his phone and then walked away. Time stamps on the footage show that he left the shrine 15 minutes before the Monday night explosion, which struck just before 7 p.m. The attack has raised concerns about safety in a city that draws millions of tourists and had never experienced an attack of this magnitude. Prime Minister Prayuth Chan-ocha "is worried about the security of people and tourists in Thailand," the police chief said Wednesday before heading into a three-hour meeting of national police commanders. Prayuth has called the attack "the worst incident that has ever happened in Thailand" and vowed to track down those responsible.

The site of the blast, the Erawan Shrine, is a revered spot among Thais and tourists that transcends religion. It is dedicated to the Hindu god Brahma, but is extremely popular among Thailand's Buddhists as well as Chinese tourists. On yesterday morning, a stream of people arrived at the Erawan Shrine, kneeling in prayer, lighting incense and placing flowers at the site where 36 hours earlier an explosion scattered body parts across one of the capital's busiest intersections. Thai authorities identified six victims as Thai and four Malaysians, along with four Chinese, two people from Hong Kong, one Indonesian and one Singaporean. Two victims remain unidentified. Bangkok was rattled by a second blast Tuesday at a popular ferry pier, which exploded in the Chao Phraya River and caused no injuries. Prawut said Tuesday's blast at the Sathorn Pier frequented by river ferries and tourist boats also was caused by a pipe bomb and could be related to the shrine attack. Security video showed a sudden blast of water over a walkway at the pier as bystanders ran for safety. Bangkok has seen politically charged violence in the past decade; the deadliest, in 2010, killed more than 90 over two months and was centered on the same intersection where Monday's bomb went off. But none of those attacks included a bomb that seemed intended to produce mass casualties.