Police officials in India’s western state of Goa received a letter last week, allegedly signed by the Islamic State group, threatening to kill Indian Prime Minister Narendra Modi and the country’s Defense Minister Manohar Parrikar. The letter is under investigation by the country’s Anti Terrorist Squad, the Press Trust of India (PTI) reported Tuesday.

A senior police official said Tuesday, according to PTI: “All the agencies of state police are investigating this letter. We will soon be able to find the source of it,” adding: “ISIS was written at the bottom of the postcard.”

The letter reportedly expressed anger over a ban against the slaughter of cows in the country — a ban that has been questioned by many after a Muslim man in a village in the northern state of Uttar Pradesh was beaten to death for eating beef. The majority of the country follows Hinduism, which considers cows as sacred.

Over the past few months, several people with suspected links to ISIS have been arrested from across the country. Last month, three men, suspected of joining ISIS, were arrested in the western state of Maharashtra.

In September last year, four Indian nationals were deported from the United Arab Emirates and later detained in India’s southern state of Kerala for suspected links to the group.

In June last year, Iran’s Defense Minister Hossein Dehghan warned that ISIS was planning to send its fighters to Central Asia, India, China and Europe. Following the warning, the group said in November that it had appointed a leader in India’s neighboring country Bangladesh.