Deputy Defence Minister General Udomdej Sitabutr said the institute’s director, Dr Weerawut Imsamran, received the letter last Friday and had already filed a complaint with police.

The letter warned of a possible terrorist attack by the southern insurgent group Barisan Revolusi Nasional and the Islamic State, which reportedly would be carried out by a Muslim woman wearing a headscarf and carrying a backpack. The attack was supposed to target three state hospitals in same area as the institute.

Nearby hospitals include Ramathibodi Hospital on Rama IV Road as well as Children’s Hospital, Rajavithi Hospital and Phramongkutklao Hospital, all of which are on Rajavithi Road. It was not immediately clear which hospitals were involved in the warning.

Meanwhile, police yesterday summoned a person of interest, Awae Yusuf, whose name and address were reportedly written on the envelope of the letter, a source said.

Police reported finding nothing suspicious after searching Awae’s residence, and he was released later in the day. The source quoted Awae as telling police that he had not sent the latter but he admitted that the address was his.

The handwritten letter written was posted from Pathumwan district.

Deputy national police chief Pol General Srivara Rangsibhramanakul said authorities had been put on alert at other hospitals in the area but not at Phramongkutklao Hospital after learning about the warning letter.

Police planned to trace the letter’s origins to find out more about it, Srivara said, adding that police asked the public for more time to collect evidence.