MOSCOW, December 12 - RAPSI. Ukrainian Deputy Prosecutor General Renat Kuzmin’s long-term US visa has been cancelled in what has been seen by some as a sign of political pressure on those spearheading the prosecution of the country’s former prime minister Yulia Tymoshenko, according to an AP report.

The report added that the US Embassy in Kiev would not elaborate on the reasons underlying the cancellation, citing privacy concerns.

Kuzmin published a letter Monday that he had written to US President Barack Obama, accusing the US Department of Justice of trying to derail a murder investigation that Tymoshenko has been implicated in.

Former Ukrainian parliamentarian and businessman Yevgeny Shcherban was shot dead at the Donetsk airport in November 1996. His wife and an airport employee were also killed.

Shcherban's son Ruslan reported in early April that he had filed documents with investigators alleging Tymoshenko’s involvement in the murder. She is presently a witness in the case. However, according to Kuzmin, the office has grounds to bring charges against her.

The former prime minister's lawyers have denied her involvement in the crime.

Kuzmin wrote that there is a strong lobby in the United States that seeks to force Kiev to stop the investigation into the murder, as well as into other criminal cases that Tymoshenko has been implicated in. In the open letter to Obama, Kuzmin added that the United States is postponing the interrogation of witnesses in the murder case, such as Pavel Lazarenko, Pyotr Kirichenko and Mykola Melnichenko, who permanently reside in the United States and have already agreed to testify.

He stressed that the U.S. Department of Justice has ignored Ukraine’s applications to hold the interrogations.

Tymoshenko has been receiving treatment at a Kharkiv hospital since May 2012. She was diagnosed with a spinal disc herniation.

In October 2011, Tymoshenko was sentenced to seven years in prison for abuse of power in connection with gas contracts she had signed with Russia in 2009. She is serving her sentence in a Kharkiv women's prison. Her lawyers have denied her involvement in the crime.

In early 2002, eight individuals were arrested on suspicion of taking part in Shcherban's assassination.

All eight were found guilty and three were sentenced to life imprisonment.