The Moscow City Court had sentenced Alexei Pichugin, 44, to 24 years in prison for two murders and organizing a murder attempt.
"The Prosecutor General's Office has filed a petition with the Supreme Court requesting that the higher court cancel Pichugin's sentence and send the case for a retrial, as the public prosecution believes the punishment for our client is too lenient," Georgy Kaganer said.
The prosecution had requested a life sentence.
Pichugin was charged with the attempted murder of businessman Andrei Rybin, and the murders of entrepreneur Valentina Korneyeva and the mayor of the Siberian oil town of Neftyugansk, Yury Petukhov.
The defense team lodged an appeal against the sentence, saying it was illegal and unfounded.
A law enforcement source said earlier that a new investigation had been launched against Pichugin, but did not give details.