NAIROBI, Kenya (AP) — Kenyan authorities are investigating the death of a man who was detained for allegedly not wearing a facemask and was seen in police custody badly beaten. Later his body was found floating in a river.

The Independent Police Oversight Authority Investigators have taken witness statements in the Lugari area of western Kenya about the arrest, disappearance and death a week ago of Dennis Lusava, 21, the group's chairwoman Ann Makori told The Associated Press on Wednesday.

The disappearance of Lusava sparked anger among local residents who set fire to a police post, a day after Lusava was arrested. His body was found last week floating in the Nzoia River, according to officials from a rights group which is also investigating the death.

It was the third police base that has been burned in Kenya since March by angry residents protesting alleged abuses by police enforcing COVID-19 regulations such as the compulsory wearing of facemasks.

Lusaka's mother Agnes Ambale said in a phone interview that she went to the police station after her son was arrested on Wednesday last week.

"When I got there they opened a room and my son was there beaten to a bloody pulp and in his underwear handcuffed to a window," Ambale said. She said the police beat him with a baton in front of her.

“I could not stand the sight and I left,” she said. Later that day she brought clothes for her son and when he could not put them on the officers beat him up some more, she said. She left in anguish again, saying the sight of her son in such condition was too painful for her. When she came back that evening the police told her that her son had been released.

The Independent Medico Legal Unit, a non-governmental organization that gives medical treatment and legal support for victims of police abuses, said their investigations confirm the account of Lusava's mother.

“We have reasons to believe that the officers at Mbururu police post who arrested him (Lusaka) and are alleged to have tortured him are responsible for his enforced disappearance while in their custody and to his eventual death,” Peter Kiama, executive director of the rights group, told The Associated Press by phone.

Kenya's police force has for two decades been ranked as the country’s most corrupt institution in public opinion polls.

In the last seven months in Kenya, 15 people, including a 13-year-old boy, have been killed by police enforcing an anti-COVID-19 curfew, according to the police oversight group. Human rights activists from the community-based Social Justice Centers put the figure at 24.

The Medico Legal Unit says 980 people have been killed by police in Kenya since 2013, and 90% of those were execution-style slayings.