Sandra Muhoza, 42, a journalist for online outlet La Nova Burundi, was arrested Sunday and is being held in the economic capital Bujumbura, family members said.
"We are very worried about the arrest of our colleague because up until now we do not know why she has been arrested," La Nova director Pascal Ndayisenga added.
The government of President Evariste Ndayishimiye, who took office in June 2020, has been widely criticized for failing to improve Burundi's dismal human rights record.
On Sunday, Burundi's Catholic bishops issued a damning assessment of the situation saying there was a culture of impunity, with killings and abductions for political or other motives.
Muhoza, a mother of three, had responded on Saturday to an invitation for an interview from a wealthy businessman who is also an influential member of the ruling CNDD-FDD party, a family member said.
The relative told AFP that after having no word from Muhoza on Saturday, her husband finally received a phone message on Sunday saying she was being held at the central offices of the intelligence service.
The family was "worried but also reassured by the message, that signified she is still alive," the relative said.
Press freedom group Reporters Without Borders, known by its French acronym RSF, said on Monday it was "concerned" about the Muhoza's detention "without any reason given."
"She's in fact being held by the intelligence services after a night at the police station," it said on X, formerly Twitter, quoting Muhoza's family.
'Journalists live in fear'
It is not the first time journalists in Burundi have been targeted.
"Burundian journalists live in fear of being threatened, attacked or arrested," RSF says on its website.
Last year, RSF ranked Burundi 114 out of 180 countries in terms of press freedom.
In 2023, journalist Floriane Irangabiye was sentenced to 10 years in prison for "undermining the integrity of the national territory," although the grounds of the charge are unknown.
In a message delivered to thousands of churches on Sunday, the Catholic Episcopal Conference of Burundi (CECAB) condemned extrajudicial killings and said there were efforts to impose a one-party system.
"It is necessary to reinforce a regime that makes room for all political groups, including those in opposition to the ruling party," the bishops said, according to a recording of the message heard by AFP.
"It is shivering to see that in our country there are people who are horribly murdered or kidnapped and reported missing for political reasons or other macabre interests."
Ndayishimiye took power after the death of president Pierre Nkurunziza, and won a subsequent election that the opposition said was flawed.
Since then, Ndayishimiye has moved to reverse years of Burundi's isolation under his predecessor's chaotic and bloody reign but continues to rule with an iron fist.
The United Nations last year voiced alarm over crackdowns on rights activists and press freedom in the country of about 12 million people.