The Security Service of Ukraine (SBU) is investigating several theories in connection with the Dnipropetrovsk explosions.
The inquiry is under way and tests are being carried out, SBU chief Ihor Kalinin told a press conference in Dnipropetrovsk.
"One of the theories we are investigating is the similarity between the November 16 (2011) blast and today’s four explosions," he said.
There are several reasons to believe all these explosions are identical: the nature of explosion, the location of a bomb, and the fact that fragments of trash containers were used as shrapnel, he said.
"If coincidences turn out to be identical based on other signs, these two cases can be put together into one inquiry," Kalinin said
As regards the November 16 explosion case, the SBU has yet to identify a suspect because the impact and the temperature of the explosion destroyed the whole evidentiary base, he said.
An unidentified explosive device detonated near a bus stop in downtown Dnipropetrovsk in the early hours of November 16. The bomb was planted in a concrete trash container. A Ukrainian man was killed in the explosion.
Initially, the criminal inquiry was launched on charges of premeditated murder, but after further investigation a regional prosecutor ruled the crime to be re-classified as terrorist.