Speaking to RFE/RL's Kazakh Service, the source, who asked not be identified, said that five businessmen were detained on March 19 and 15 the following day.
Ablyazov was sacked from his position as the head of BTA, one of Kazakhstan’s largest and most stable banks, after the government bought the majority of the bank's shares in February.
He and several associates had to leave Kazakhstan after investigations were officially launched into their alleged financial misdeeds.