The raid at the For Latvia From The Heart (NSL) party office took place on October 4, party leader and former state auditor Inguna Sudraba confirmed to LTV.
Sudraba said she was not on the premises at the time and did not know whether the KNAB officers had taken anything away, but insisted she was confident the party had done nothing wrong and kept good accounts.
"I see no reason to believe in any illegal activity linked to the party," Sudraba told LTV.
According to information obtained by LTV, one of those detained was businessman Julijs Krumins, believed to be a major donor to the party via his daughter Maija Čerņavska, who is also a former business partner of Sudraba.
According to the LETA news agency, another person detained was former Parex Bank executive Jorens Raitums and another businessman, Eriks Igaunis.
The raid appeared to be linked to another raid October 4 at the offices of the Jurmala municipality and mayor Gatis Truksnis of the Greens and Farmers Union party, which is part of the ruling coalition.
Truksnis was reportedly hospitalized after the raid, but is believed to have reported to KNAB's HQ on Wednesday.
NSL on the other hand is an opposition party with just 4 MPs. It surprised many people by claiming the mandates in the 2014 parliamentary elections, partly thanks to a big-spending advertising campaign that attracted questions at the time as to where all the funding was coming from for a start-up party.
Also in connection with the KNAB investigation, Riga court judge Aija Ornina was reported to have stepped down from her position. She said she was not at liberty to disclose information about an ongoing investigation but confirmed the move was liked to the arrest of Krumins.