An audio clip, purportedly from an Islamic State in Iraq and Syria (ISIS) fighter, has surfaced, with a message that encourages sympathisers to carry out attacks at religious gatherings.
Its discovery has raised fears of lone-wolf attacks in the South Asian country.
The clip in Malayalam, the language spoken in the southern state of Kerala, calls for Las Vegas-type terror attacks at crowded and popular festivals such as the Kumbh Mela and Thrissur Pooram, said reports in the Indian media.
A Kerala district police chief said they are trying to ascertain if the 10-minute clip is related to the arrest of five ISIS sympathisers last month in Kannur, one of 14 districts in the state.
The Kumbh Mela is the world's largest religious gathering with tens of millions of pilgrims taking a dip in the river Ganges to wash away their sins, while the Thrissur Pooram is a popular annual Hindu temple festival held in Kerala.
A Kerala police official said the clip is being investigated by state and federal law enforcement and intelligence officials.
It came to the notice of the authorities as it was being shared on multiple WhatsApp groups in Kerala.
Media reports say it features a man's voice, saying: "Use trucks. Drive over them at Thrissur Pooram or at Maha Kumbh Mela. Islamic State Mujahideen are doing it in several parts of the world. In Las Vegas, one of our supporters killed many people at a music concert."
The American gunman had shot dead 58 concertgoers last month.
>90 Number of ISIS sympathisers who have been arrested in the last couple of years, according to the Indian home ministry.
Kerala, where 26 per cent of the population are Muslims, is among India's more prosperous state. Religiously diverse, it has seen a more conservative strain of Islam emerging in the last few years.
Said Kannur Police Chief G. Siva Vikram: "This clip is being shared on many WhatsApp groups. We cannot take it lightly."
At the same time, the authorities are ascertaining its authenticity, he added. "We do suspect a person. Investigations are on."
The National Investigation Agency and other state and federal agencies are investigating as well, he said.
ISIS does not have a direct presence in India nor has been successful in large-scale recruitments from India. But there have been reports of young people from the states of Kerala, Jammu and Kashmir, Tamil Nadu and Kerala travelling to Syria and Iraq to join the group.
India's home ministry said more than 90 ISIS sympathisers have been arrested by its security agencies in the last couple of years.
Lately, the government has said it has intelligence linking some Rohingya Muslims in India to ISIS and expressed concern that they could carry out terror attacks in India.
Analysts said lone-wolf attacks remained a threat, but given the quantum of ISIS activity in India, it is not significant.
Said Dr Ajai Sahni of the Institute for Conflict Management in New Delhi: "In the last 21/2 years, there have just been arrests except for one low-intensity bomb in a train."
He added: "We should be more worried about Pakistan-based terror groups."