Bangkok, Thailand (CNN) Police are looking for a "woman wearing a black shirt" in connection with this week's deadly bombing at Bangkok's Erawan Shrine, a senior police spokesman said Thursday.

"I would urge her to come forward to provide information to police," Lt. Gen. Prawut Thavornsiri told reporters without elaborating.

Earlier, two men captured on surveillance video entering the shrine ahead of Monday's bombing turned themselves in, but it does not appear they're linked to the attack, Prawut said.

The men -- a tourist and tour guide described by the colors of the shirts they were wearing at the shrine: white and red -- were taken to metro police headquarters for interrogation, said Col. Decha Promsuwan, a police official in Bangkok. They both had been sought by police but have denied any connection to the alleged bomber, he said.

Later, Prawut told reporters it was unlikely they were involved in the blast.

At least 10 people may have taken part in the bombing, but the attack is unlikely to be linked to international terrorist groups, Thai authorities say.

Police have singled out one main suspect, describing him as an unidentified foreigner who was caught on surveillance video hiding a backpack under a bench at the shrine minutes before the bomb detonated.

Royal Thai Police Commissioner Gen. Somyot Poompanmoung said Thursday he believes "several teams" were involved in the preparation and execution of the attack, which killed at least 20 people and wounded more than 120.

"This operation was carried out by a big network," he said.

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

"There must be a preparation for materials and explosives," Somyot said. "There must be people who scout the route. There must be people who survey the site, people who would cover and look after the bomber. There must be people who know (the) escape route and take the bomber to do it."

Even though police say at least one foreigner may have been involved in the attack, the operation is unlikely to be linked to international terrorism, Col. Winthai Suvaree, a spokesman for Thailand's ruling military junta, said Thursday.

There has been no claim of responsibility for the bombing. Thai authorities haven't said what they think the motive for it might have been, other than making vague references to unspecified people or groups who want to hurt Thailand's tourism industry and economy.

Information shared with Interpol

Thai security forces are sharing information with Interpol, the global police organization, and with intelligence agencies from allied countries, officials said.

At this point, they don't know if the main suspect is still in Thailand, said Prawut, the police spokesman.

He said investigators are reviewing surveillance video from Bangkok's two international airports as well as from the scene of the bombing.

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

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

Police on Thursday doubled the reward for information leading to the suspect's arrest to 2 million Thai baht ($56,000), according to the government-run National News Bureau of Thailand. They raised it further Friday to 3 million baht ($84,000), Somyot said.

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

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

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

Police say they have questioned the motorcycle taxi driver but haven't provided details of what he told them.