Suggestions that a man with links to violence in the South was involved have further muddied the waters.

The Department of Special Investigation (DSI) yesterday denied leaking a photograph identifying a man implicated as one of three suspects in the attacks.

DSI director-general Sunai Manomai-udom said his agency should be the only one involved in the investigation. It is armed with special powers authorising it to second members of other agencies and overseas personnel, operate telephone taps and infiltrate crime syndicates.

Prime Minister Surayud Chulanont asked the DSI to jointly investigate the case with the Crime Suppression Division (CSD) and the Metropolitan Police Bureau (MPB). Last week the police were forced to release military and civilian suspects after failing to produce sufficient evidence to charge them.

Sunai will meet with the DSI board and if it "agrees this is an emergency case the DSI can immediately take over the case without waiting for a decision to be made at next month's regular meeting," he added.

A mug shot of a man - identified as Ramkhamhaeng University dropout Thawalsak Paenae - was leaked to the news media on Thursday. It is said the man is wanted by police in Yala's Betong district for alleged involvement in bomb attacks at several provincial banks late last year.

Sunai said DSI agents spotted a man "similar in appearance" to Thawalsak in video footage from surveillance cameras at Seacon Square. The man was observed depositing a potato-crisp container into a refuse bin that later exploded.

"I insist here I am one-million-per-cent confident the leak did not come from the DSI. I don't understand why the photo of this man - which has been in the possession of three agencies for a month - was released only after the DSI began jointly investigating the case," Sunai added.

He said that while the man's surname indicated his origin and birthplace as the strife-torn South there was no direct link between his recorded insurgent activities and the bomb attacks in Bangkok.

Sunai was certain that more than three people involved in one of the New Year's attacks were caught on video recordings.

First Army Region Lt-General Prayuth Jan-ocha said the military, police and DSI investigators had to rethink strategies now the photograph had been leaked.

He said Council for National Security chairman General Sonthi Boonyaratglin had given him orders that nothing in the case be "distorted".