NEW DELHI (Reuters) - Indian federal investigators launched a fraud probe on Monday into the founders of news channel NDTV, in a step the channel denounced as an attempt to muzzle free speech in the world’s largest democracy.

Two senior officials at the Central Bureau of Investigation (CBI) said it had registered a case against NDTV founders Prannoy Roy and his wife, Radhika Roy, for causing an alleged loss of 480 million rupees ($7.45 million) to a bank.

The company refuted the allegations and accused the CBI of “concerted harassment of NDTV and its promoters based on the same old endless false accusations”.

“NDTV and its promoters will fight tirelessly against this witch-hunt by multiple agencies. We will not succumb to these attempts to blatantly undermine democracy and free speech in India,” NDTV said in a statement.

The investigation comes at a time of often heated, polarizing debate in India’s fiercely competitive TV news industry, where channels sometimes test the boundaries of responsible journalism and prime-time talk shows are often rowdy and contentious.

Police conducted raids at Roy’s residence in New Delhi and at two vacation homes.

Neither of the couple were available for comment but NDTV presenter Srinivasan Jain said the “message is clear”.

“Any independent voices in media will be bullied and shut down. Black day,” tweeted Jain.

The CBI said the investigation had no link to the editorial line of NDTV, India’s oldest English-language all-news channel.

“We have nothing against NDTV’s news coverage ... they are accused of a financial fraud and its our duty to investigate,” a senior CBI official said.

India slipped by three places to 136th in this year’s World Press Freedom Index compiled by Reporters Without Borders, which highlighted concerns that Hindu nationalists were “trying to purge all manifestations of anti-national thought”.

Senior officials in Prime Minister Narendra Modi’s office and members of his nationalist BJP have told Reuters they consider NDTV to be the least-government friendly channel.

In a heated exchange last Thursday, an NDTV moderator demanded that a spokesman of the ruling Bharatiya Janata Party (BJP) either apologize or leave their live debate after he accused the channel of pursuing an anti-government agenda.