They were told that only Deputy Prime Minister General Prawit Wongsuwan or national police chief Chakthip Chaijinda would be allowed to talk to the media, the source said.

However, Chakthip yesterday declined to give any comment on the case, saying the investigation team was working on it.

The hospital blast probe team recently found a person who may be a key witness in the case. A nurse said she saw a suspect carrying a backpack into the building shortly before the explosion, the source said.

They hoped to get confirmation on the person seen by the nurse, which would allow them to apply for an arrest warrant, the source said.