Two people who were identified as suspects in the lethal Monday bombing at a shrine in the Thai capital of Bangkok turned themselves in to police on Thursday, before being released later in the day.

The two were caught on security camera footage just before the explosion at Erawan Shrine sitting on a bench at the scene. They got up just before the main suspect sat down and left behind a backpack that apparently held the bomb.

Both men insisted they are tour guides and that their presence at the shrine was innocent; they were questioned by authorities before being released.

Police have said at least ten people are suspected of being involved in the bombing, and identify the main suspect as being a foreigner. Further details regarding his identity remain unclear, and no international terror group has yet claimed responsibility for the attack.

No less than 20 people were murdered in the blast, 12 of them foreigners including visitors from China, Hong Kong, the UK, Indonesia, Malaysia and Singapore. Many more were wounded in the explosion.

Several hours earlier on Thursday, Col. Winthai Suvaree of the Thai army said it is “unlikely” the attack was conducted by a foreign terrorist group. However, shortly afterwards he told Associated Press that the terror angle is still open, saying, “we still have to investigate in more detail.”

Thailand’s army chief on Monday hinted the attack may have been the work of a global terrorist organization, when he said it did not match the tactics used by separatist Muslim rebels in southern Thailand.

“This does not match with incidents in southern Thailand. The type of bomb used is also not in keeping with the south,” Royal Thai Army chief and deputy defense minister General Udomdej Sitabutr said in a televised interview.

More than 6,500 people have been murdered in the long-running Muslim insurgency since 2004.