General Panlop Pinmanee and General Chaisit Shinawatra, who are viewed as opposing the ruling junta, maintained that they had nothing to do with the explosion at the military-run hospital.

Panlop, a former deputy director of the Internal Security Operations Command (ISOC), said that throughout his career as a professional soldier, he was well aware of the rule about never attacking a hospital even in a time of war.

“I had been in battlefields throughout my military career – from the rank of second lieutenant to full general. In war, we never do harm to hospitals or medical staff. We know the rule well,” Panlop said.

In condemning the perpetrators, he said: “They are wicked, vicious and cowardly.”

Panlop also said that he was not acquainted with militant red-shirt leader Wuthipong “Ko Tee” Kotchathammakhun, who was described by some military commanders as a suspect in the hospital bomb attack. More tha

n 20 people were injured in the bomb blast, some of them seriously.

Chaisit, previously the Army commander-in-chief and armed forces supreme commander, said that the allegation against him was groundless, adding that he had not been involved in politics for a long time.

“I definitely have nothing to do with the three bomb incidents in Bangkok,” he said, referring to the hospital attack and two other recent explosions in the capital.

The former Army chief added that he was a regular at Phramongkutklao Hospital for medical check-ups.

Chaisit is a cousin of former prime minister Thaksin Shinawatra. He was appointed to the top military posts when Thaksin was in power.

Earlier, a newspaper report said retired generals with the initials P, C and S were behind the hospital bomb blast.

Deputy national police chief Pol General Srivara Ransibhramanakul yesterday said this was merely a possibility and that so far it was not being focused on by investigators.

Srivara, who is heading the police probe into the hospital blast, said the team would focus on collecting evidence for scientific proof, instead of relying on allegations or speculation.

The investigators were zeroing in on a man suspected of placing the explosive device inside the hospital, according to Srivara. He added that images of the suspect obtained from security camera recordings were not clear and that officials were attempting to identify him before an arrest warrant would be issued.

Meanwhile, forensics experts have determined that the same person wrote all three letters warning of a possible bomb attack on a hospital before Monday’s blast.

The letters warned that Muslim insurgents from the deep South might stage a revenge attack against a medical institution in Bangkok.

A police source said yesterday that two of the letters were sent to the director of the Prasat Neurological Hospital and Institute and the third to the director of the National Cancer Institute.

The source said forensic examination found that the same person wrote all three letters, though that person had attempted to disguise his or her identity by modifying the handwriting and content.

The letters were written on the same type of paper, which under magnification indicated the writer had traced the wording from one letter to the next, applying the same amount of pressure.