One man, a Thai national, was questioned by authorities on Thursday evening. The pair was later cleared of involvement in the blast after investigators were told he was a tour guide.

The second man, a tourist from China, has already left Thailand, officials said.

Suspicion about the two men emerged after police released grainy security camera footage showing them standing in front of the prime bombing suspect, wearing red and white T-shirts.

The pair got up from a bench at the Erawan shrine just before the suspected bomber sat down, later leaving his backpack behind.

On Wednesday, investigators had requested that the two men come forward if they were innocent to rule themselves out of the inquiry into Monday's bombing, which left 20 people dead and scores wounded.

But there was confusion as to their involvement after separate video footage from a nearby street appear to show them spooked by the sound of the explosion and had jumped to seek cover. This suggested that they had no connection to the bombing, detectives added.

The two men have not been named.

Police said on Thursday they had identified the rickshaw (Tuk Tuk) driver who picked up the main bombing suspect close to Bangkok's main Hua Lamphong railway station on Monday evening and dropped him at the Ratchaprasong intersection, near where the blast happened.

Police have questioned the driver but think the suspected bomber may have left the country.

He's been described as a tall, fair-skinned foreigner speaking neither Thai nor English. CCTV footage showed a young man with glasses, wearing a yellow t-shirt.

Officials confirmed on Thursday they were hunting a fourth person in connection with the bombing.

A woman dressed in black was seen on a security video at the time of the explosion.

Having consulted with foreign intelligence services, investigators think the blast was the work of at least 10 people and involved around a month's planning.

A Bangkok-based analyst told DW on Thursday that Thai authorities may have more information about the bombing, which they've yet to release publicly.

mm/kms (AP, dpa)