In another development, a group of youngsters destroyed the fence of Maj-General Chamlong Srimuang's restaurant "Suan Phai" on Thursday night.

Chidchai said: "The investigation is progressing okay but it is too early to set any deadline for completing it."

The evidence from the latest case will be examined along with past bombings at the Santi Asoke religious centre and Phujadkarn editorial office, he said.

Forensic checks on the explosive used were being carried out separately by police and military labs.

Sources said the bomb was professionally assembled, hinting that men in uniform might be involved.

Although police had jurisdiction over the investigation, Chidchai has appointed a top-level panel to supervise it, including senior civilian, military and police officials from the National Intelligence Agency, Armed Forces Security Centre, and Metropolitan Police.

A senior military intelligence officer said the type of TNT explosive used was a strong lead to help narrow down suspects, although their motive is still a mystery.

"Bombing masterminds would obviously benefit from the incident but at this juncture, it is still unclear who, among the supporters and opponents of the government or a third party, would gain the most from the explosion," the officer said.

At the Metropolitan Police Region 1 Office, investigators took a statement from retired Sergeant Pornlert Chaloliew, who was seen arriving at the vacant security booth in front of Prem's home 10 minutes before the explosion.

He helped a police artist sketch the profile of a young man whom he saw on the other side of the booth but who left minutes before the blast.

The young man had some scars on his neck, which could be old shrapnel wounds, he said.

Metropolitan Police commissioner Lt-General Wiroj Chantha-rangsri went to inspect the scene for a second time. He ordered his men to repair the guard booth, install security cameras and increase the lighting at the gate to Prem's home.

Army chief General Sonthi Boonyaratklin instructed the Army Ordnance Department to trace the explosives.

Top officials were seen at Prem's home giving moral support in the wake of the bombing.

Prime Minister Thaksin Shina-watra, after visiting Prem to convey his apology and a promise to apprehend the culprits, said he condemned all forms of violence.

"Culprits for the bombing will be arrested regardless of who they are," he said.