Thai soldiers inspect the scene after a bomb exploded outside a religious shrine in central Bangkok late on August 17, 2015.[Photo/CFP]

"We've seen two of them, frequently. One was the arrested man, but there's another, he's much taller," said the man, who requested anonymity because he feared for his safety.

The detained man was reclusive but always appeared focused and walked with intent on his rare forays outside. They said he was often seen on his knees praying outside the room.

"I still fear danger," the woman said. "We don't know if the other man has been arrested."

Speculation has focused on which groups could have motive and capability to carry out the bombing.

On Saturday, police indicated the person arrested was the prime suspect, a young man with shaggy dark hair who wore a yellow shirt and was picked out on security cameras dropping off a backpack at the shrine and leaving before the bomb went off.