Tanzanian police have arrested a prominent investigative journalist and say they are investigating his citizenship amid increasing concerns about the safety of reporters in the east African country.

Erick Kabendera, who has written for the Guardian and the East African among other publications, was detained on Monday at his house on the outskirts of the commercial capital, Dar es Salaam, by a group of men who claimed to be plainclothes police officers, according to witnesses.

The men refused to identify themselves, disconnected the house’s internal phone system and took the family’s mobile phones, Kabendera’s wife, Loyce, told the Tanzanian newspaper the Citizen, which called the incident an “abduction”. The Citizen said the men had forced their way into Kabendera’s home, demanding to search it.

The men said they were taking him to Oysterbay police station, but he did not appear to have gone there on Monday night and was not in the central police system, according to colleagues and a lawyer who went to check on him.

On Tuesday, the Tanzanian police’s inspector general, Simon Sirro, confirmed that Kabendera had been taken to a police station for questioning. “Police identified themselves and arrested him by following all proper procedure. He is still under police custody,” Sirro said.

Later, a police spokesman told reporters that they were working with immigration officials. “We’re continuing to discuss his case with the office of immigration, and we’re handing it over to them to clarify his citizenship,” said the spokesman, Lazaro Mambosasa.

“After his arrest, we heard various reports saying he had been abducted. I should say that the journalist was not kidnapped. He was called [to the police station] but he aggressively rejected the call.”

Kabendera’s parents were mistreated during interrogations over allegations that they were not Tanzanian citizens in 2013. After he and the NGO Reporters Without Borders complained, the immigration officials responsible were disciplined and the ministry of home affairs said there was no question over their citizenship.

The New York-based Committee to Protect Journalists (CPJ) said it was concerned about Kabendera’s safety.

“The manner in which this journalist was taken, by men claiming to be police, is very ominous and further evidence that the press is not safe in President John Magufuli’s Tanzania,” said Muthoki Mumo, the CPJ’s sub-Saharan Africa representative, who was detained in the country last year.

Since Magufuli won the 2015 election, repression has increased in Tanzania. Newspapers have been shut down and journalists driven into exile. A number of opposition members have disappeared, opposition politicians have been arrested, and a local opposition leader was found beaten to death after being kidnapped last year.

In recent weeks Kabendera has published stories about infighting in Tanzania’s ruling party, Chama Cha Mapinduzi, and an alleged plot to block Magufuli from running for a second term next year.

The CPJ said a journalist close to Kabendera who asked not to be named had spoken with him just before the arrest and he expressed concern for his safety.

Earlier this month Tanzania said it did not know whether another journalist who went missing two years ago while investigating a series of murders of police and ruling party officials was dead or alive.

Activists have cited the disappearance of Azory Gwanda in November 2017 as a sign of worsening conditions for journalists under Magufuli’s government, which they say has cracked down on press freedoms by suspending newspapers. The government rejects the criticism.

In 2009 Kabendera won a David Astor award, named after the former Observer editor, which helped young east African journalists gain experience at British newspapers, but which were suspended in recent years owing to lack of funding.