Four Bulgarian interior ministry officials were indicted on Tuesday over the alleged wiretapping of outgoing premier Boyko Borisov's political opponents, Sofia prosecutors said.
The head of the ministry's surveillance unit Sergey Katsarov and his two predecessors Tsvetan Ivanov and Kamen Kostov were charged with disregarding their duties and facilitating unregulated wiretapping dating back to 2009.
Another official was being probed for trying to cover up the case by allegedly destroying the illegal recordings.
Under Bulgarian law, technical surveillance is only allowed in investigations of serious crimes and all four officials face jail sentences if found guilty.
Prosecutors opened a probe last month after the head of the opposition Socialists, Sergey Stanishev, asked them to verify an anonymous report alleging the widespread use of wiretaps on Borisov's rivals.
The report accused interior minister Tsvetan Tsvetanov of ordering the communications monitored of former president Georgy Parvanov and the leaders of the ultra-nationalist and Turkish minority parties during their dealings with Borisov's GERB party.
Other suspected victims include former finance minister Simeon Djankov, European Commissioner Kristalina Georgieva and several influential businesspeople.
Tsvetanov has denied the accusations. They come at an awkward time for him since he is the campaign manager for Borisov ahead of elections on May 12.
Borisov resigned on February 20 after large-scale street protests over poverty and corruption that also saw an unprecedented seven people set themselves on fire in the European Union's poorest country.
Analysts also said the scandal was unlikely to influence voters ahead of the elections in a country wearily used to tales of surveillance since the end of communism in 1989.
The high number of cases where surveillance was carried out but never used as evidence in court was highlighted in a US State Department report earlier this month.