Naghiz Ak Zhol opposition party leader Sarsenbaev was found dead on February 13, along with a bodyguard and a driver.
Opposition leaders say they believe Sarsenbaev's assassination was political and commissioned by high-ranking government officials.
Utembaev, the main defendant at the trial, was heading the Senate's administration at the time of the killing.
He was said to have admitted to ordering the murder in a letter to President Nursultan Narzarbaev but later recanted that purported confession.
Kazakhstan introduced a moratorium on executions in December 2003.
(with additional reporting by Interfax-Kazakhstan and Kazakhstan Today)