Still image from closed-circuit television (CCTV) footage released by Thai police shows what they believe to be two new suspects - dressed in red (C) and dressed in white (3rd R) - standing in front of previously identified supect in yellow t-shirt removing his backpack prior to a bomb blast in Bangkok, made available August 19, 2015. [Photo/Agencies]

BANGKOK - Police in Thailand issued an arrest warrant on Wednesday for an "unidentified foreign man" shown in a security video leaving behind a backpack just minutes before a deadly bombing at a Bangkok shrine, but acknowledged they're not sure he's from another country.

Police also released a sketch of the suspect and offered a 1 million baht ($28,000) reward for help leading to the arrest of the man they believe carried out Monday's bombing, which left 20 people dead and more than 120 injured. Authorities have called it the worst attack in Thai history.

But apart from the rough portrait that shows a young man with dark, bushy hair and eyeglasses, authorities have few solid leads.

National police spokesman Lt. Gen. Prawut Thavornsiri expressed uncertainty at a news conference about the man's origin. He said the suspect "looks like a foreigner" but "might have been in a disguise and wearing a fake nose" to conceal his identity.

Prawut said the security video appears to show two possible accomplices standing in front of the man who are also considered suspects.

"He didn't do it alone, for sure. It's a network," national police chief Somyot Poompanmoung told the same news conference, adding, "I believe there are some Thais involved."

Two days after the attack, the open-air Erawan Shrine reopened to the public. No one has claimed responsibility for the blast at one of the capital's busiest intersections during evening rush hour.

Authorities identified the suspect from a grainy security video that shows the man, wearing a yellow T-shirt and carrying a black backpack at the crowded shrine. He sits down on a bench, takes off the backpack and leaves it behind as he stands up and then walks away. Time stamps on the video show he left the shrine 15 minutes before the explosion, which struck just before 7 p.m.

"If citizens or anyone can give us information or clues that lead to the arrest of this man, I have set a reward of 1 million baht," Somyot said.

Police composed the sketch based on the video and a description provided by a motorcycle taxi driver believed to have given him a ride on Monday night.

The arrest warrant describes him as tall, with a pointed nose and thick lips. He faces six charges including conspiring to commit premeditated murder and conspiring to commit a bombing that resulted in death and severe injuries.

The attack has raised concerns about safety in a city that draws millions of tourists.

Prime Minister Prayuth Chan-ocha "is worried about the security of people and tourists in Thailand," the police chief said.

Prayuth has called the attack "the worst incident that has ever happened in Thailand" and vowed to track down those responsible.

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.