Two Nigerian journalists have been charged in court over the killing of nine female polio vaccinators in northern Kano state on Friday.
They were charged with conspiracy and inciting a disturbance.
Their Wazobia FM radio station had aired the views of people opposed to polio vaccinations in the mainly Muslim north two days before the killings.
The New York-based Committee to Protect Journalists reportedly said it was "troubled" by the prosecution.
Some Muslim leaders in northern Nigeria believe polio vaccinations cause infertility among women.
They see it as a Western conspiracy to reduce the Muslim population.
Such opposition is a major reason why Nigeria is one of just three countries where polio is still endemic.
There were 121 cases of polio in Nigeria last year, compared to 58 in Pakistan and 37 in Afghanistan, according to the Global Polio Eradication Initiative.
No group has said it carried out Friday's two separate attacks on the polio vaccinators.
Some have accused Islamist militant group Boko Haram of the killings but it has not commented on the allegations.
In the first attack in Kano, vaccinators were shot dead by gunmen who drove up on a motor tricycle.
Thirty minutes later gunmen targeted a clinic outside Kano city as the vaccinators prepared to start work.
Kano police chief Ibrahim Idris earlier told the BBC the journalists would face a charge of culpable homicide.
But during a court hearing in Kano, prosecutors brought lesser charges that included conspiracy, inciting a disturbance and obstruction of a public servant, AP news agency reports.
Wazobia's station chief Sanusi Bello Kankarofi told AFP news agency that a third journalist was released after being questioned by police.
He said a man who featured on their popular Sandar Girma programme, and was allegedly forced to submit his children to vaccinations by district officials, was also arrested.
Mohamed Keita, an official with the Committee to Protect Journalists in New York, said the group was "troubled" by the arrests as there appeared to be no evidence linking the programme to the killings, AP reports.
"We call on Nigerian authorities to afford the journalists due process under the law,'' he is quoted as saying.
Nigerian journalists often complain about harassment by police and secret security service agents.