AHMEDABAD, India — The two men were unlikely candidates to work in the news business.

Neither had a background in journalism, but both were alarmed with the surge of misinformation in India that followed the rise of Narendra Modi as the Hindu nationalist prime minister. To take on this problem, the men, both engineers, started Alt News in 2017.

Since then, Alt News has become a leading fact checker in India, debunking rumors on social media that often spiral onto television news, including those about child-kidnapping gangs and that Muslims were spreading Covid. Calling out hate speech has also become part of the site’s work as it has taken aim at viral posts that inflame sectarian tensions and that sometimes spur violent mobs to attack innocent people.

Led by its founders, Mohammed Zubair and Pratik Sinha, Alt News has criticized supporters and officials of Mr. Modi’s Bharatiya Janata Party for their statements targeting minorities.

But in a reflection of the growing concerns about the independence and freedom of the news media in India, Mr. Zubair has landed in the authorities’ cross hairs. He has been arrested on charges of hurting religious sentiments and is being investigated by the police after anonymous critics and B.J.P. officials accused him of spreading communal unrest.

“People in power want to shut me up for exposing their propaganda, their lies and their hate campaigns,” Mr. Zubair, 40, said in an interview. “They want to scare other journalists and activists by targeting me.”

Mr. Zubair, a Muslim, said that rather than amplifying misinformation and hate speech, he was trying to highlight them so the authorities could take action. Still, he worried for his family’s safety this summer as #arrestzubair trended on Twitter. He temporarily stopped his children from riding their bicycles outside and from going to school.

The media landscape in India started to change when Mr. Modi came to power in 2014. His party realized the potential of reaching voters directly via social media and spent millions of dollars to mold public perception on platforms like WhatsApp and Facebook.

Critics say that engagement, and later copycat efforts from other political parties, lacked the filter of a traditional news organization and targeted millions of people who were using the internet for the first time.

“I could also see that propaganda was building up and how misinformation was part of that,” said Mr. Sinha, then a software engineer in Ahmedabad in the western state of Gujarat, who started debunking misleading photographs. He was not the first person in his family to take on Mr. Modi’s acolytes; his parents were activists who had faulted Mr. Modi for not doing enough to stop violence against Muslims in the deadly Gujarat riots of 2002, when he was chief minister of the state.

Around the same time in Bangalore, Mr. Zubair, an engineer from a family of farmers, was also taken aback by the increasing spread of misinformation among Indians. His first attempt at tackling the problem was with satire, creating a social media account that was a parody of a leader of India’s governing party. His musings attracted an audience, and soon he crossed paths with Mr. Sinha.

They set up an office in Mr. Sinha’s home in Ahmedabad, slowly expanding their team to more than a dozen people, including fact checkers with media backgrounds and video producers. Mr. Sinha’s mother, Nirjhari Sinha, who had run a nonprofit, also joined Alt News, which publishes content in English and Hindi and has drawn about 80 million pageviews so far. It is nonpartisan and nonsectarian.

“They are a consistent and thorough voice in myth-busting and fact-checking,” said Anant Nath, the editor of Caravan, a monthly magazine. “They are doing very important work establishing public accuracy.”

Alt News was among more than a handful of websites that emerged in India in the past decade to counter misinformation that was spreading on the internet. “The mainstream media had failed to counter these narratives,” said Jency Jacob, the managing editor of BOOM, another Indian fact-checking website. “In India, disinformation often takes the worst turn towards building hate against certain vulnerable sections of society.”

And the work is only getting harder. Government data show a nearly threefold rise in India of instances of “fake news” and “rumors” from 2019 to 2020, even as social media companies take steps to curb the spread of misinformation. Mr. Sinha, 40, said that although it was tough finding where much of the bad information originated, all political parties used it to further their political agendas. “But the party benefiting most from it was the B.J.P. — that much one can say,” he said.

Earlier this year, Mr. Zubair highlighted comments made by a B.J.P. official that many Muslims considered as insulting to the Prophet Muhammad. The incident became a diplomatic embarrassment for Mr. Modi after Muslim nations demanded an apology, which the official did offer. Mr. Zubair has also called out extremist Hindus who urged the killing of Muslims.

A B.J.P. spokesman, Tom Vadakkan, rejected the assertion that the party benefited from misinformation, adding that Alt News itself has faced similar accusations. In response, the founders of Alt News say they are providing a public service by debunking bogus claims.

Some critics say Alt News staff members aren’t journalists — four of 14 staff members have journalism degrees — and point to Mr. Zubair’s opinionated and sometimes abrasive tweets as evidence. This summer, Mr. Zubair was jailed after an anonymous Twitter user, citing a tweet from 2018, accused him of insulting a Hindu god and hurting religious sentiments. He was eventually granted bail but only after being dragged from courtroom to courtroom, paraded before the news media in a pile of cases that made it difficult to win release. He is still being investigated in that case and for other complaints that he caused unrest with his tweets.

But despite this turmoil for Alt News, which is run by the founders’ nonprofit company, Pravda Media Foundation, supporters have stood by it. Donations that fund the site have remained steady, through contributions that average $12 a month per person.

“What’s helping Alt News sustain are small donations. This is what scares them,” Mr. Sinha said of the site’s critics.

Control of the Indian news media in India has come under scrutiny again after a hostile bid for New Delhi Television, an independent outlet known as NDTV. Many observers fear a compromised editorial line if the bidder, Gautam Adani, Asia’s richest man and a friend of Mr. Modi’s, captures a majority stake.

At Alt News, the work continues as usual despite the cases pending against Mr. Zubair. The organization has bought an office in Ahmedabad and has opened a bureau in Kolkata. A book on misinformation is in the pipeline.

At the Ahmedabad office one recent morning, Mr. Zubair, Mr. Sinha and the rest of the team huddled to discuss which news and information to track, prioritizing whatever might have the potential to cause harm. They scoured WhatsApp groups for leads. Mrs. Sinha worked with an accountant on Alt News’s finances.

Nearby, another employee, Kinjal Parmar, replayed a viral video of a mob beating a man viciously, frame by frame. Soon she reaffirmed the conclusion her co-workers had reached: The footage was of a personal dispute, not of a Muslim man’s lynching. Next, she posted an article on the Alt News site that corrected the record, reducing the chances that the video would inflame communal tensions.

Ms. Parmar, who trained as a journalist, said no special skills were needed to be a fact checker, except an eye for spotting what’s amiss. She said the work was a mission for her.

“Our job entails providing every citizen the right to correct information,” she said. “And in times of so much fake information, it becomes all the more important in a democracy like India.”