Bangkok, Thailand (CNN) The chief suspect in the deadly bombing of Bangkok's popular Erawan Shrine is "an unnamed male foreigner," according to an arrest warrant issued Wednesday by a Thai court.

A Thai motorbike taxi driver who believes he picked up the suspect shortly after the blast also said he did not seem to be Thai.

Driver Kasem Pooksuwan, 47, told CNN that the man -- who spoke an unfamiliar language on his cell phone during the short ride -- didn't speak to him at all but showed him a piece of paper with the name of a central city park written in English.

"When I dropped him, he still appeared very calm, just like (a) normal customer. He seemed not in a hurry at all," Kasem said.

Thai police spokesman Lt. Gen. Prawut Thavornsiri earlier said police had questioned the motorcycle taxi driver but did not give details of what the man had told them.

Police believe the man suspected of carrying out the deadly bombing in the heart of the Thai capital probably had accomplices.

Examination of surveillance video footage from the Erawan Shrine indicates at least two others were helping the main suspect in the attack that killed 20 people and wounded more than 120 Monday evening, police officials said at a news conference Wednesday.

Authorities are hunting for the man seen on a surveillance video putting a backpack under a bench in the shrine and then walking away shortly before the blast went off.

Beyond the yellow T-shirt and dark-framed glasses he was wearing, little is known so far about the man who police say they are "very sure" is the bomber.

Thai Prime Minister Prayuth Chan-ocha appealed on Wednesday for the chief suspect to turn himself in to authorities. Addressing reporters, Prayuth said he feared the suspect "might be silenced by killing" and that if he wanted to be safe, he should surrender.

Here's where things stand with the investigation into the attack, which authorities have described as a deliberate act of terror:

The chief suspect

Police say they don't yet know the suspect's identity but do now appear to have concluded that he's a foreigner.

They are studying more than 10 days' worth of closed-circuit TV footage from the scene.

The surveillance footage shows the suspect in the yellow shirt sitting down on a bench in the shrine at 6:52 p.m. Monday and hiding the backpack under the bench, said Prawut, the police spokesman. The man left without the backpack, and the blast went off minutes after he set it down.

Police have released a sketch of the suspect, who is described as a dark-haired man with glasses and light facial hair.

Thai police have released a sketch of the suspect in the shrine bombing.

Police also said the suspect had material wrapped around his forearms, which they say could indicate he had previously suffered some kind of injury.

A reward of 1 million Thai baht ($28,000) is being offered for information leading to the suspect's arrest, he said.

Kasem told CNN he thinks the suspect "is not Thai," although he admitted he did not look at his whole face. He said he remembers a long chin and white pale face very well, however.

"I wish I looked at his face more, but I really didn't know what happened yet at that time. Normally I would not stare at my customer's face," he said.

Kasem, who has been a Bangkok taxi driver for a little more than two years, said he did not think the unfamiliar language spoken by the man was English. He dropped his passenger at Lumpini Park, by which time he was no longer on the phone, he said. It wasn't clear whether he placed the call or received it.

The suspected accomplices

After studying surveillance footage from the shrine, police have zeroed in on two men they think could have been working with the man in the yellow shirt, said Prawut.

One was wearing a red shirt and the other a white shirt. The pair can be seen standing in front of the chief suspect as he places his backpack under a bench shortly before the explosion.

Police want to speak to both men as to whether they stood there to shield the yellow-shirted man's actions from view.

The device

The blast was caused by a pipe bomb containing wheel bearings, according to police.

Photos: Explosion in Bangkok Photos: Explosion in Bangkok Thai security officials escort bombing suspect Yusufu Meerailee, arrested last week near the Cambodian border, as he is transferred from military to police custody at the metropolitan police headquarters in Bangkok on September 7, 2015. Hide Caption 1 of 39 Photos: Explosion in Bangkok Thai authorities unveiled the repaired centerpiece of the Erawan Shrine on September 4, 2015 to restore confidence among Bangkok's tourism and business communities almost three weeks after a deadly bomb attack on August 17. Hide Caption 2 of 39 Photos: Explosion in Bangkok Visitors gather around the base of the Erawan Shrine on August 24, 2015 to commemorate the victims of the bomb attack in Bangkok, Thailand. Hide Caption 3 of 39 Photos: Explosion in Bangkok Monks pray at the Erawan Shrine on August 24 to commemorate the victims of the bombing. Hide Caption 4 of 39 Photos: Explosion in Bangkok A coffin of one of the 20 victims killed in the blast is sent home on Wednesday, August 19, 2015. Hide Caption 5 of 39 Photos: Explosion in Bangkok Family members of Neoh Hock Guan ,a Malaysian survivor of the Bangkok bomb blast, break down as they pay their last respects. Five out of the seven Neoh family members traveling to Bangkok together were killed in the blast. Hide Caption 6 of 39 Photos: Explosion in Bangkok Foreigners offer prayers at the reopened Erawan shrine. Authorities say the perpatrators targeted busy places popular with tourists to damage the economy. Hide Caption 7 of 39 Photos: Explosion in Bangkok A man offers incense and prays at the Erawan shrine. Hide Caption 8 of 39 Photos: Explosion in Bangkok At least 10 people are believed to have taken part in the bombing, but the attack is unlikely to be linked to international terrorist groups, Thai authorities say. Hide Caption 9 of 39 Photos: Explosion in Bangkok This image released by the Royal Thai Police on August 19 shows a detailed sketch of the main suspect in the bombing. Hide Caption 10 of 39 Photos: Explosion in Bangkok Messages and flowers for victims killed in the bomb blast are displayed outside the shrine. Hide Caption 11 of 39 Photos: Explosion in Bangkok Damage from the blast is seen on the statue of Phra Phrom at the Erawan Shrine on August 19. Hide Caption 12 of 39 Photos: Explosion in Bangkok Workers make repairs to the buildings in and around the Erawan Shrine on August 19. Hide Caption 13 of 39 Photos: Explosion in Bangkok Workers build a dam from sandbags on August 19 as they attempt to seal off a canal to search for remnants of an explosive device that was thrown into the canal in Bangkok on Tuesday. Police spokesman Lt. Gen. Prawut Thavornsiri said Tuesday's blast at the Sathorn Pier was caused by a pipe bomb and could be related to the deadly explosion at a popular shrine in central Bangkok on Monday. Hide Caption 14 of 39 Photos: Explosion in Bangkok Authorities have put down fresh cement where a bench used to stand and where they believe the bomb detonated. Thai authorities are searching for a suspect who was seen placing a backpack underneath the bench moments before the explosion. Hide Caption 15 of 39 Photos: Explosion in Bangkok Family members gather around the coffin of a bombing victim on Tuesday, August 18. The death toll from the bomb blast in the Thai capital stood at 20 with more than 120 wounded, police said. Hide Caption 16 of 39 Photos: Explosion in Bangkok A Thai lights a candle near the Erawan Shrine the day after the explosion in Bangkok. Hide Caption 17 of 39 Photos: Explosion in Bangkok Police investigators work near the statue of Phra Phrom, the Thai interpretation of the Hindu god Brahma, at the Erawan Shrine the morning after the explosion. Hide Caption 18 of 39 Photos: Explosion in Bangkok Thailand's prime minister on Tuesday promised that authorities would quickly track down those responsible for the central Bangkok bombing. Hide Caption 19 of 39 Photos: Explosion in Bangkok Police and soldiers wash the street in front of the Erawan Shrine. Hide Caption 20 of 39 Photos: Explosion in Bangkok Glass covering an advertising billboard is shattered. Hide Caption 21 of 39 Photos: Explosion in Bangkok The explosion took place at the busy Rajprasong intersection near the Erawan shrine but also international hotels and shopping malls. Hide Caption 22 of 39 Photos: Explosion in Bangkok Policemen inspect the cordoned-off site of a bomb blast in the heart of Bangkok's tourist and commercial center on Monday, August 17. Hide Caption 23 of 39 Photos: Explosion in Bangkok Police dogs search the scene of the explosion. Hide Caption 24 of 39 Photos: Explosion in Bangkok Authorities transport motorcycles destroyed in the blast. Hide Caption 25 of 39 Photos: Explosion in Bangkok Rescue workers carry the body of a victim away from the scene. Hide Caption 26 of 39 Photos: Explosion in Bangkok A wounded person lies on a gurney at the Police General Hospital in Bangkok. Hide Caption 27 of 39 Photos: Explosion in Bangkok Police officers watch security video footage. Hide Caption 28 of 39 Photos: Explosion in Bangkok A Thai Army officer collects evidence in the street. Hide Caption 29 of 39 Photos: Explosion in Bangkok Emergency response personnel inspect the scene of the explosion August 17. Hide Caption 30 of 39 Photos: Explosion in Bangkok A police officer investigates the scene at the Erawan Shrine. Hide Caption 31 of 39 Photos: Explosion in Bangkok Police take photos at the scene of the explosion August 17. A bomb exploded in front of the shrine shortly after 7 p.m., a news report said. Hide Caption 32 of 39 Photos: Explosion in Bangkok Police enter the Erawan Shrine after the explosion. The shrine sits at a busy intersection, with the city's Skytrain rumbling nearly overhead. Hide Caption 33 of 39 Photos: Explosion in Bangkok A bomb disposal team member suits up in the middle of an intersection after the blast. Hide Caption 34 of 39 Photos: Explosion in Bangkok A police officer addresses the media after the explosion. Hide Caption 35 of 39 Photos: Explosion in Bangkok Destroyed motorcycles lie among debris after a blast near a popular Hindu shrine in Bangkok on August 17. Hide Caption 36 of 39 Photos: Explosion in Bangkok A Thai soldier stands guard at the scene of the blast. Hide Caption 37 of 39 Photos: Explosion in Bangkok Thai soldiers inspect the scene of the explosion on August 17. Hide Caption 38 of 39 Photos: Explosion in Bangkok Rescue workers carry an injured person after the bomb exploded on August 17. Hide Caption 39 of 39

Investigators say they think the device was assembled in Thailand because many parts, including the pipe, were made in the country, Prawut said.

CNN security and intelligence analyst Bob Baer said it was unusual for a pipe bomb to kill so many people and cause so much damage.

"It would be a very sophisticated pipe bomb," he said.

Another blast that struck near a river pier in Bangkok on Tuesday was caused by a similar device, Prawut said. That explosion went off in the water and didn't hurt anyone, but it was still "quite big," he said.

Royal Thai Police Commissioner Gen. Somyot Poompanmoung said Wednesday that investigators were so far unable to establish a link between the two explosions, adding they have no leads on who might have been behind Tuesday's blast.

The motive

There has been no claim of responsibility for the shrine bombing, and comments from Thai officials so far have been vague.

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"From this incident, it is apparent that there are active individuals or groups that harbor the intention to damage Thailand, who may be pursuing political gain or other intentions by damaging the economy and tourism," the Prime Minister said Tuesday.

Analysts have speculated about a wide range of groups -- including opponents of military rule in the country or a rogue faction in the army or police -- but there is so far no evidence to link the attack with any of them.

Asked if there was any information on a possible motive, police spokesman Prawut said earlier this week it was too soon to reach any conclusions.

The victims

Many of those killed were from other countries.

The shrine is a popular tourist site and sits at a busy intersection near a big shopping mall.

China's state-run news agency reported that six Chinese citizens, two from Hong Kong, were killed. Malaysia's national news agency Bernama said that four Malaysian citizens, all from the same family, were among the dead.

Officials have said that five Thais, a Briton and a Singaporean also died.

Police had put the overall death toll at 22, but on Wednesday they revised it down to 20, saying confusion arose from the number of body parts scattered at the scene.

The shrine

Monks in orange robes chanted and prayed for departed souls Wednesday at the shrine, which bore the scars of the powerful blast.

A golden statue of the four-headed Hindu god Brahma was missing most of one of its jaws. Fresh concrete had been poured over the crater caused by the explosion.

Tributes of flowers were piled on a table and pinned to the railings outside. "Rest in peace" read one paper sign attached to a barrier.

Some people prayed, while others took photos.

The site is popular among Buddhists as well as Hindu and Sikh members of Thailand's Indian community.