GENEVA (AP) — Swiss federal prosecutors said Tuesday they have ordered house searches and interrogations as part of an investigation into possible electoral fraud after an advocacy group unearthed thousands of fake signatures on voter rolls.

The allegation that malfeasance that could have skewed results in Swiss elections has jolted Switzerland’s vaunted system of direct democracy, where mail-in balloting is the main way that voters cast ballots and where referendums four times a year give citizens a direct say in policy-making.

Newspapers in Swiss media group Tamedia first reported the criminal complaint filed by the “Service Citoyen” (Citizen Service) group.

The next referendum in Switzerland is scheduled for Sept. 22. Advocacy groups and others that want to float proposals for referendums must collect at least 100,000 signatures from eligible voters to qualify for national votes, and at times turn to outside firms to do so.

The advocacy group's co-president Noemie Roten commissioned an outside firm last year to collect some 10,000 signatures on voter rolls. She discovered that nearly a third appeared fake, involving bogus addresses, falsified birth dates and repeated signatures.

She issued a 236-page report to the Swiss attorney general's office, which said in an email to The Associated Press on Tuesday it is investigating the matter.

In a phone interview, Roten said the findings “could indeed jeopardize the confidence that Swiss citizens have in direct democracy.”

The Swiss federal chancellery, which oversees national elections, said it was carrying out increased checks of cantons, or regions, from which it had received evidence of forged signatures. It said it could not make any reliable assessment about how widespread the practice was pending the investigations.