He said the unsigned papers were believed to have come from Muslim fighters blamed for scores of attacks across Thailand's south in recent years.
On Friday three Muslim boys were shot dead by Thai troops after the soldiers said they came under fire.
The killings sparked street protests among Thai Muslims who form the majority in the three southern states of the largely Buddhist country.
On Sunday, suspected insurgents bombed a roadside restaurant in Pattani township, leaving one customer wounded.
However, local Muslim villagers stressed they had nothing to do with the attacks and had agreed to stop protests.
"It must be the work of insurgents who are taking advantage of the situation... to create chaos," said Marueding Jehka, a representative for the families of the boys killed, told the Associated Press.
Colonel Akara Thiprot, a spokesman for the Thai army, said it was likely the fighters were seeking to incite more anger in the Muslim community after the issue had been resolved.
"The soldiers have confessed their guilt and apologised," Akara said.
He said the army has punished the soldiers and agreed to pay compensation to parents of the victims and "both sides are satisfied with the agreement."
In separate violence on Monday morning, two Muslim men who worked as drivers for a local government office in Yala province, were shot dead.
Police say they believe the two were targeted because they worked for a government office and were seen as collaborators.