State Attorney General Bob Ferguson says the companies violated state campaign finance law, prompting him on Monday to seek injunctions for the companies' alleged failure to disclose ad spending in state elections since 2013.
Washington candidates and political groups reported advertising payments of about $3.4 million to Facebook and $1.5 million to Google in the last decade, according to the state Public Disclosure Commission.
The state's campaign finance laws require commercial advertisers that sell political advertising to maintain information about those who purchase it, and the advertisers are required to make that information publicly available.
"Washington's political advertising disclosure laws apply to everyone, whether you are a small-town newspaper or a large corporation," Ferguson said in statement. "Washingtonians have a right to know who's paying for the political advertising they see."
Facebook says it's in contact with Ferguson's office and actively explaining its ad transparency features.
"The tools we are introducing set a new standard for transparency in digital advertising. We are eager to hear people's feedback as they use these features and will continue to explore how to build upon them to ensure people know who is behind the political ads they see on Facebook," Rob Leathern, Facebook's director of product management, said in a statement.
"Attorney General Ferguson has raised important questions and we look forward to resolving this matter with his office quickly."
Google said it's "committed to transparency and disclosure in political advertising."
"We are currently reviewing the complaint and will be engaging with the Attorney General's office," a spokesperson wrote.
Facebook and Twitter on May 24 added verification, disclosures and additional information to political ads on their platforms.
Update, 8:32 a.m. PT: Adds Google statement.
Fight the Power: Take a look at who's transforming the way we think about energy.
'Hello, humans': Google's Duplex could make Assistant the most lifelike AI yet.