On June 5, Kwanpetch Sudthes, 11, and Kwanmanee Sudthes, 10, suffered fatal symptoms of poisoning while riding bicycles to shop for their grandmother Srinuan.

Kwanpetch died instantly, while his sister went into a coma and succumbed to organ failure. She died the next day.

The Lampang Centre Hospital's autopsy reports later revealed the children's stomachs had traces of "Lannate" pesticide.

Police discovered Srinuan was the beneficiary of payouts after two previous poisoning deaths in the family - her husband in 2004 and another grandchild earlier this year.

Lt-Colonel Panas Saithong of Sop Prap police station yesterday invited Srinuan to hear the charge of premeditate murder filed against her, as circumstantial evidence pointed to her as the culprit.

She had a motive to kill the children because their deaths meant Srinuan, as the sole beneficiary, would "earn" nearly Bt200,000, police alleged.

A police probe into Srinuan's financial situation found she owed Bt95,000 to the Bank for Agriculture and Agricultural Cooperatives' Sop Prap branch and had debts with the Village Fund. The bank had urged her to repay debts this month.

Police are waiting for test results on 25 items of evidence submitted to the Forensic Institute at Chiang Mai's Maharat Hospital. If traces of poison are found, police will seek court approval to detain Srinuan as a murder suspect and not grant her bail, because the crime had shaken public morale and captured media attention.

Meanwhile, Ban Pongka village headman Thananetr Supati said the village's committee had agreed not to give Srinuan the funeral costs for her two grandchildren pending the police investigation into the suspicious deaths.