Modification of the explosives in all three cases was similar and probably done by people with knowledge on ordinance or military training, he said. "Ordinary [untrained] people cannot do it," he said.

In the bomb attack in front of Big C Rajdamri, the M67 hand grenade was modified to multiply the effect of the blast, before it was placed in a cardboard box in a black garbage bag and put on a trash pile next to a busy bus stop near Big C.

On May 14, a large cache of home-made weapons including Molotov cocktails [petrol bombs] and time- bombs was found abandoned in the Khok Khram police jurisdiction of northern Bangkok, and a home-made bomb was planted in an area of Nang Lerng police on April 3.

Amnuay said the way the explosives in all three incidents were modified was similar, but did not explain further.

Police would "have a clearer picture" by today over whether the man seen in security footage putting a black bag on the garbage pile was a suspect, and if the case was deemed a case of terrorism, it would be taken over by the Department of Special Investigation.

Meanwhile, a fake bomb was planted yesterday in a telephone booth near Soi Sathu Pradit 34 in Bang Phong Phang district of Bangkok. Police ordinance personnel opened a plastic bag and found an electronic beeper wired to a pair of batteries.