The Criminal Court on Wednesday approved arrest warrants for two suspects who were seen planting bombs on the walkway between Siam BTS station and Siam Paragon on Sunday.
The first suspect, wearing a white shirt, a white cap and carrying a backpack, is seen in CCTV footage near the blast sites dropping an item at the Bangkok Metropolitan Administration (BMA) one-stop service centre, said national police spokesman Pol Lt Gen Prawut Thawornsiri.
The second one, wearing a white long-sleeved shirt and a black cap, is seen placing another item behind a billboard of the department store where another bomb went off, he said. The warrants do not name the suspects.
The pair are facing charges of premeditated murder, planting a bomb and carrying weapons in public places.
There is no clear information as to whether the pair entered the BTS skytrain system, Pol Lt Gen Prawut said.
He said the suspects were seen loitering near the blast sites shortly before the bombs went off.
More than two people are believed to have been involved in the explosion, the spokesman said, adding the attacks were coordinated and well planned.
Police are still inspecting CCTV footage looking for more suspects.
Pol Gen Chakthip Chaichinda, deputy national police chief, said the two suspects are highly likely to be the bombers.
"I believe the perpetrators are skillful," the deputy police chief said. "They appeared calm as they carried the bombs."
Pol Gen Chakthip repeated the police theory there is a link between the Siam blasts and two other explosions, at a Min Buri car park in March last year and the Samarn Metta Mansion in Nonthaburi's Bang Bua Thong district in October 2010.
The blast at the Min Buri car park killed two men who police believed carried a home-made bomb in their motorcycle. The device went off accidentally.
The explosion inside the apartment killed a red-shirt guard and three other victims.
The probe suggests Sunday's bombs could come from the same source as similar bomb containers and other elements were found in all three attacks, Pol Gen Chakthip said.
He said police have discounted the motive the blast was caused by a rift between vocational students.
"I have always said there was an 80-90% chance the blasts stemmed from politics," Pol Gen Chakthip said. "Similar incidents in the past were politically motivated."
He also warned more attacks could be launched if assailants spot an opportunity.
He said the perpetrators hoped to achieve various objectives from the blasts as the attack happened in the heart of the city.
Pol Gen Chakthip said police have obtained CCTV footage showing how the suspects left the scene after the blasts.
Meanwhile, Department of Special Investigation director-general Suwana Suwanjuta yesterday said the agency has handed to police its files on the blasts at the Min Buri car park and Samarn Metta Mansion, which it has taken as special cases for investigation.