At least two parcels containing eight M67 grenades and bullets from the same sender were reported at the Kerry Express branch on Phahon Yothin Road in Bangkok's Bang Khen district after the privately owned courier could not find its recipient.
Bang Khen police were called to the courier branch on early Friday afternoon to examine bomb-like objects found in a parcel sent on May 6.
The sender was listed as Issarapong Prombutr with no detailed return address except Bangkok. The recipient was Mr Mag of No.33/1 Moo 6, Kid-ngoeng district, Phanas Nikhom, Chon Buri province, who could not be located.
Ratchaneekorn Rujiprach, a Kerry employee, told the police that the box was returned to the point of origin after seven days with no recipient.
Employees at the shop on Friday opened the parcel and found suspicious bomb-like objects. They reported to the police and sought help from explosive ordnance disposal officers.
The EOD officers found four ready-to-use M67 grenades inside the box.
Later on the same day, Thai media quoted deputy police spokesman Pol Col Krissana Pattanacharoen as saying another parcel by the same sender was found at the branch. It contains another four M67 grenades and some bullets.
Col Piyapong Klinphan, a National Council for Peace and Order spokesman, said the investigation found the sender was a corporal and asked people not to panic.
"This is just a mail delivery, not a preparation for an unrest. It could be a sale or a threat to someone. We're investigating," he said.
Possession of explosives is illegal and the penalty doubles for government officials, who will also face disciplinary action, he said.