Demonstrators in New Delhi demanding justice in September after the death of the Indian journalist Gauri Lankesh.

Tsering Topgyal/Associated Press

NEW DELHI — When an Indian journalist was shot and killed outside her home one warm night in September, news of her death reverberated across the country.

The connection between the death of Gauri Lankesh and her work — which was often sharply critical of India’s governing party — was not lost on observers. Rahul Gandhi, an opposition leader, wrote on Twitter at the time that anybody who “speaks against” the Bharatiya Janata Party and Hindu nationalist groups affiliated with it are “attacked & even killed.”

That theory was back into the spotlight on Friday when the authorities detained Naveen Kumar, the founder of the far-right group Hindu Yuva Sena, according to Bijay Kumar Singh, the leader of a special team formed to investigate Ms. Lankesh’s death.

“He is in our custody for interrogation,” Mr. Singh said by telephone on Saturday.

During her career, Ms. Lankesh, 55, had occasionally been sued after being accused of defamation by leaders of the Bharatiya Janata Party. The journalist, who lived alone in the city of Bangalore, in southern India, was known as a “rationalist,” a term in India for people who stand against superstition and the use of religion in politics.

When the Bharatiya Janata Party rose to power in 2014 with the election of Narendra Modi as prime minister, hard-line groups connected with his party amplified calls to transform India into a “Hindu nation” by shrinking space for religious minorities.

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