"A man suspected of involvement in the murder of a Russian citizen in Biała Podlaska has been detained," Tusk wrote on social media, crediting police and officers from the Internal Security Agency (ABW) for the arrest.
He added that the suspect was carrying a Georgian passport and that authorities were working to establish "who ordered the killing."
Police said the suspect was detained on Thursday morning near Warsaw during a joint operation involving a special investigative unit from the Lublin regional police headquarters, counterterrorism officers and ABW agents.
Andrzej Fijołek, a spokesman for police in the regional capital Lublin, said the man was carrying a passport issued to a 36-year-old Georgian citizen.
"We will be working to establish his full identity," Fijołek said, adding that investigative procedures involving the suspect were ongoing.
The investigation was initially handled by the Lublin District Prosecutor's Office, but prosecutors said on Thursday that the case had been taken over by the National Prosecutor's Office.
According to investigators, the killing took place on Monday morning on a sidewalk in a residential area of Biała Podlaska, a city near Poland's border with Belarus.
An assailant approached the victim and fired two shots from a handgun. After the victim fell to the ground, the gunman is believed to have moved closer and fired three additional shots at close range.
The scene of the fatal shooting in the eastern Polish city of Biała Podlaska on Monday, June 15. Photo: PAP/Wojtek Jargiło
A postmortem examination found seven gunshot wounds, including injuries to the head, chest and back.
The victim was identified as Russian citizen Robert K., who was publicly known as Semyon Skrepetsky, also rendered as Simon Skrepetski. Media reports described him as a dissident artist and outspoken critic of Russian President Vladimir Putin.
Speaking on Wednesday, Tusk said all indications suggested the killing may have been politically motivated, though investigators still needed evidence to support that conclusion.
"If it turns out that this was a murder ordered by Russia, it would be a very serious matter with international implications," Tusk said. "That would be state terrorism," he added.
(gs)
Source: TVP Info, IAR, PAP