Ukrainian police have detained five people, including a paediatric surgeon and a rock musician, over a 2016 car bomb attack that killed the investigative journalist Pavel Sheremet.

Senior law enforcement officials said the motive for killing 44-year-old Pavel Sheremet was to destabilise the country but declined to say who might have ordered the murder.

“In this country, very few people believed that such a crime could be solved. This is the first step in solving this crime, because only those who planned and committed the murder have been identified,” the prosecutor, Gen Ruslan Ryaboshapka said.

“The country still needs to hear the answer to who was the initiator and organiser of this terrible murder.”

President Volodymyr Zelenskiy said: “The probable killers were detained today. But there is another question: who ordered it?”

Police suspect at least five people were involved, and said all had served as volunteers in Ukraine’s war against Russian-backed rebels in the eastern Donbass region. A sixth suspect had been invited for questioning but killed themself, they said.

None of the five suspects made statements. One opposition lawmaker said the detentions appeared an attempt to repress people who were defending Ukraine.

The five included a paediatric surgeon who in 2016 was given a state award for her humanitarian assistance in the east. She planted the bomb under the car, police said, along with a rock musician who was also an army volunteer.

Sheremet, originally from Belarus, was killed in Kiev on his way to work. He was known for his criticism of his home country’s leadership and his friendship with the Russian opposition leader Boris Nemtsov, a critic of the Kremlin who was murdered in 2015.

In 2002, he won a journalism prize from the Organization for Security and Co-operation in Europe (OSCE) for his reporting on human rights violations in Belarus, including the disappearances of opposition politicians and journalists.

He was given Russian citizenship after fleeing persecution in Belarus but relocated permanently to Ukraine in 2014 after no longer feeling safe living in Moscow.

The murder has been a source of criticism of Ukraine’s interior ministry and security services, which made no progress on the case for years and classified parts of it, which invited suspicions of government involvement.

Western diplomats, the OSCE and rights activists had urged Ukraine to solve the case.

Sheremet worked for the online news website Ukrayinska Pravda, whose founder, Georgiy Gongadze, was also murdered in 2000. His decapitated body was discovered in a forest outside Kiev.