Retired Army Sergeant Pornlert Chaloliew, 50, said he had seen a young man of apparent southern descent with scars on his neck sitting near the vacant security booth for 20 minutes drawing a diagram before leaving the scene moments before the blast.

Reading the news yesterday morning about a bomb suspect who sounded like himself, cooking-gas store employee Nikhom Saen-on, 23, went to the Sam Sen police station in the afternoon to clear his name.

He told reporters he had been at the scene because a Sam Sen policeman investigating an accident he had had on November 14 had asked him to draw a plan of the incident, which left him with the neck scars and a broken front tooth.

Sitting near the guard booth from 12.30pm to 1pm as he was drawing, Nikhom said he had seen Pornlert talking with a tall, dark man aged about 30 to 35, wearing a short-sleeved brown shirt, jeans and a pair of sneakers.

The war veteran offered the other man a drink, which he refused, saying in central Thai dialect that he had quit drinking five years before. Nikhom said he did not know what had happened after that because he had then left the scene.

Police took Nikhom into an investigation room while reporters waited outside.

Earlier yesterday, Colonel Preecha Thimamontri, head of the metropolitan police investigation team, said police were going over the bomb scene for every possible shred of information as it was a tough case.

The bomb was home-made and different to the ones used by southern insurgents, he said.

The investigators will hold a meeting early this afternoon with Metropolitan Police Commissioner Lt-General Wiroj Chantha-rangsri, he said. The demolished security booth is being rebuilt and will be completed in 20 days.

While police and Army officers boosted security around Prem's residence, Director-General of the Signals Department Lt-General Nipon Theerapong said on Friday he would seek funds from the Army to install security cameras around the premises for better protection.