The executive board of Rigas Satiksme, the municipal transport company of the Latvian capital city, resigned on Thursday after Latvian anti-corruption operatives made several arrests this week as part of a probe into bribery and money laundering related to the company's procurement deals.

According to a statement released by the Latvian Corruption Prevention Bureau, the criminal investigation concerns several Polish and Czech companies' unlawful dealings with representatives of the Riga city council and Rigas Satiksme, involving bribery and money laundering.

The Polish and Czech suppliers of passenger vehicles allegedly had been paying the bribes to Riga city council officials and Rigas Satiksme managers in order to secure contracts in three tenders held in 2013 and 2016.

In 2013, Rigas Satiksme organized a tender to purchase new passenger buses for 75.8 million euros, and another tender was held the same year to buy trolleybuses for 131.6 million euros. In 2016, the municipal transport company announced a tender to purchase low-deck tram cars for 62.6 million euros.

Latvian media reported on Wednesday that eight arrests were made and over 30 searches conducted this week in Latvia and Poland as part of a sweeping probe.

Representatives of the Corruption Prevention Bureau informed that the case has been investigated since November in close cooperation with foreign colleagues, including Poland's anti-corruption service CBA, and that eight arrests -- six in Latvia and two in Poland -- were made on Tuesday as part of the probe.