The Bulgarian Football Union (BFU) has asked the state prosecutor and the Interior Ministry to investigate allegations of match-fixing involving several top-flight games last week.
Local media reported there were unusual betting patterns before a number of matches, including Vihren's 4-1 win over Belasitsa Petrich on Friday and relegation-threatened Spartak Varna's 2-1 win at Slavia Sofia on Saturday.
Some club officials were quoted by media as saying they "knew the outcome of some games before they even started".
"The BFU's ethic commission discussed the media reports pointing to foul play in some first division matches," the BFU said in a statement on Thursday.
"The BFU sent a letter to the state prosecutor and to the Interior Ministry, asking for assistance."
The Balkan country's championship has been marred by frequent accusations of bias among match officials with Levski Sofia's owner Todor Batkov saying last year there had been match manipulation and illegal betting in recent seasons.