The suspects, Russian citizen Adam Osmayev and Kazakh national Ilya Pyanzin, had previously been accused of illegally handling of arms and explosives when the case was opened on March 12. But the charge has been changed to "creation of a terrorist organization and preparation for a terrorist attack," Ukrainian Security Service spokeswoman Marina Ostapenko said.
Ukrainian Prosecutor General spokesman Yuri Boichenko said Russia had inquired about extraditing Osmayev. He said prosecutors needed to conduct an "extradition test" and attain a court order before Osmayev could be sent to Russia.
Kazakhstan has not made inquiries about having their citizen returned, Boichenko said.
Ostapenko said the case is being examined by the security service's head investigative committee but said it is too early to comment on where and when the trial will take place.
"We need to conduct an investigation before we can talk about the trial," she said.
The two men were shown confessing to the plot in a video aired on Russian state TV on Feb. 27, in what some believed to be a stunt to boost Putin's ratings a week before the presidential election.