Newspapers in the north-eastern Indian state of Tripura have run blank editorial pages to protest the murder of a journalist this week, the third prominent killing of a media worker in India since September.

Suddip Datta Bhaumik, a crime reporter, was allegedly shot dead on the orders of an army official on Tuesday while investigating a story on a paramilitary base.

Police have accused Tapan Debbarma, the commandant of the Second Tripura State Rifles, of ordering his bodyguard to shoot Bhaumik. Both have been arrested.

One of Bhaumik’s editors told local media the reporter’s “only crime” was visiting the base to investigate a story about financial irregularities within the battalion.

Most of the newspapers in the state ran blank editorial pages on Thursday as an “institutional protest”, said Manas Paul, the editor of the Tripura Times.

“[The] murder of two journalists within a month is a matter of serious concern,” he told the news agency ANI.

Another journalist, Santanu Bhowmick – no relation to reporter killed this week – was abducted and murdered by a mob while covering unrest between tribal groups in the state in September. His mutilated body was found the next day.

Gauri Lankesh, an editor and activist, was shot dead on her doorstep the same month by armed assailants, a killing that sparked national protests because of her prominent opposition to the rightwing ideology of India’s ruling Bharatiya Janata Party.

The Karnataka state chief minister has formed a Special Investigation Team to find Lankesh’s killers and promised they will “100% be caught”.

Critics of the government allege the space for dissent in India is shrinking and that violent fringes of the Hindu nationalist movement are being indulged or emboldened by BJP state and central governments.

Civil rights groups have flagged the misuse of sedition charges and increasing numbers of people arrested for social media posts criticising politicians, as well as the journalist murders, as signs of growing hostility to free expression in the world’s largest democracy.

Prime minister Narendra Modi last week said his government was committed to “upholding the freedom of the press and expression in all forms”.

Bhaumik, 50, reported for the Bengali daily Syandan Patrika and the TV channel Vanguard.

The Committee to Protect Journalists (CPJ) said in a report last year that 27 journalists – excluding the three this year – had been killed “with complete impunity” in India since 1992. It listed another 25 murders it was investigating to ascertain a connection to the journalist’s work.

The CPJ ranks India 13th in its Global Impunity Index, highlighting countries where the murders of journalists are least likely to be punished. The organisation claims not a single journalist’s murder in the country has been solved in the past 10 years.

The journalists considered most at risk are those working outside major cities who report in languages other than English and focus on crime and corruption, particularly in the lucrative resources industry.