Two men pulled off the daring daytime robbery in Chiang Saen district on May 18 but were then tracked down by the police.
One of them, Sitthichoke Meuangma, a 34-year-old former Chiang Saen agricultural cooperative employee, gave himself up soon after. Police said the man confessed to robbing Bt1.05 million with his brother-in-law to pay off debts.
The brother-in-law, Cheunjai Konkham, 48, was found dead by the police in the Phayao forest. Police assumed he committed suicide.
Pol Maj Gen Thammasak Pinthong, provincial police general, said that a forensic team has collected evidence and sent the body for an autopsy to verify the cause of death.