Alexander Buksman said one of the versions was "linked with Kozlov's professional activities."
Kozlov, 41, died Thursday morning after gunmen opened fire on him with semi-automatic weapons late Wednesday. Kozlov's driver was also killed in the attack.
Buksman said 30 people are involved in the investigation of Kozlov's murder. "They are the best investigators and most qualified employees," he said adding that no suspects had been detained yet, and that their photokits were not available.
He also said Kozlov's killers were professionals judging by a number of details. Buksman said the gun's handle "was wrapped with textile tape that leaves neither fingertips nor smell."
Buksman said according to one of the four versions, Kozlov could have been killed by mistake, but it was a minor version.
Kozlov oversaw the Central Bank's efforts to clean up the banking sector, and was shot in an attack that bears the hallmarks of a contract killing as he left a sports stadium in the northeastern Moscow district of Sokolniki.
His driver Alexander Semyonov was killed at the Spartak stadium, where Kozlov had watched Central Bank employees play soccer. The banker underwent emergency surgery for a gunshot wound to the head.
But a hospital official said Thursday morning that doctors were unable to save the married father of three after an operation lasting five hours.
President Vladimir Putin offered his condolences to the banker's family Friday and asked the Central Bank's heads "to make every effort to support them."
Kozlov led efforts to close down dozens of banks for violations of banking legislation, particularly on money laundering, and Central Bank regulations. Two high-profile cases centered on the revocation of licenses from Moscow-based Sodbiznesbank in 2004 and Neftyanoi Bank this year, but the CBR has been withdrawing licenses almost by the week in 2006.