A woman identified as a mistress of Watana Pumret, the chief suspect in the May 22 bombing at Phramongkutklao Hospital, was likely involved in the attack as well as others to which he had confessed, a security source said Sunday.
"The security authorities aren't totally convinced Mr Watana acted alone as he claims. Over the five years of his bomb attack operations, it is believed he received assistance from others, or that his moves were known by others," said Col Burin Thongprapai, an army staff judge advocate.
He said soldiers have completed questioning Mr Watana, a 62-year-old retired engineer, and will hand over him to police tomorrow.
Meanwhile, the security source said authorities have targeted his mistress as among those people likely to have given Mr Watana support in attacks.
"His mistress is found to have strong political preferences toward one political camp and every time before Mr Watana carried out those attacks, he would go to meet his mistress at a condo in Nonthaburi," said the source.
"This has also led security officials to suspect that there are possibly more people involved."
Mr Watana, who was detained on Wednesday night, faces five warrants in connection with the Bangkok blasts.
One fact that added weight to the theory that more people were involved in the attacks is that Mr Watana's handwriting proved to not resemble any of that found in three letters sent to warn the National Cancer Institute of Thailand and Prasat Neurological Institute about a possible bomb attack, the source said.
The letters were sent to the two institutes that are in the same neighbourhood as Phramongkutklao by anonymous senders, some time before the May 22 attack at Phramongkutklao.
Mr Watana was told by investigators to write more than 10,000 words and his handwriting was examined and compared with those in the letters, the source said.
Mr Watana, however, remained firm in his previous statement that he acted alone, motivated by his strong political ideology against the military coup and his deep dislike of the military for the soldiers' roles in the May, 2010 dispersal of the red-shirt protesters at Wat Pathum Wanaram in Pathumwan district of Bangkok under the last Abhisit Vejjajiva government, the source said.
Security authorities found no link between Mr Watana's wife (the registered wife) and the bomb incidents although some war weapons and explosives were found at the wife's house in the Ram Intra area, the source said.
She was a retiree with no known political preferences who has provided useful information to investigators in their probe, the source said.
A probe of the financial transaction history of Mr Watana's only bank account showed no signs of irregularities, but the investigators believe there still are more bank accounts to be found, the source said.
During a marathon interrogation, Mr Watana made a new confession regarding the bombing attack outside the old GLO, said the source.
He added the suspect offered more details as to how he travelled to plant a pipe bomb there and fled.
