A Ukrainian court on Tuesday arrested a man suspected of murdering an activist lawyer who helped convict a Kiev judge's nephew for killing her sister in September 2015.

Yuriy Rossoshansky, 64, was placed in custody for 60 days over the murder of 38-year-old Iryna Nozdrovska and told the examining magistrate he was guilty, saying "I agree with the prosecution".

Nozdrovska was reported missing at the end of last month and on January 1 her body was found in a river in the Vyshhorod district near Kiev.

For two years she was working on the case against Dmytro Rossoshansky, Yuriy Rossoshansky's son, who was driving under the influence and hit her sister by the car.

He was sentenced in June 2017 to seven years behind bars.

On Monday, just a week after Nozdrovska was found dead, Ukrainian police announced the detention of a suspect, but Nozdrovska's family have questioned the results of the investigation.

Their attorney Oleksandr Panchenko told Ukrainian media that Rossoshansky is just a "scapegoat" for the crime.

"For us the information about this suspect (Rossoshansky) is very contradictory at the moment," Panchenko told journalists before the court hearing.

Ukrainian President Petro Poroshenko thanked police for their "brilliant work and a quick reaction" in a case that sparked public outrage.

The high-profile case was seen a test of the Ukrainian justice system's ability to prosecute people linked to the upper echelons of power who had seemed untouchable prior to the pro-EU revolution that swept Kiev in 2014.

A US embassy statement last week commented on the case, indicating it was "shocked and saddened by the death of activist Iryna #Nozdrovska. We offer our deepest condolences to her friends and family. Those responsible must be brought to #justice."

Ukrainian Foreign Minister Pavlo Klimkin last week called the killing "a challenge to the state"