Special counsel Robert Mueller has subpoenaed a former top employee of Cambridge Analytica, the defunct Trump-campaign-linked data firm.

Brittany Kaiser, the former business development director for Cambridge Analytica, was subpoenaed and is cooperating with Mueller's team, her spokesperson told the Guardian.

The data firm, which once had former Trump strategist Steve Bannon as its vice president, worked on President Trump's 2016 campaign, but collapsed last year after it was revealed the company acquired the personal information of nearly 90 million Facebook users through an online personality test.

Cambridge Analytica, launched with the help of Republican megadonors Robert and Rebekah Mercer, also worked with advocates of the United Kingdom's departure from the European Union, resulting in interest from British parliamentary opponents.

Mueller's probe, widely seen as nearing a conclusion, is looking for possible Trump campaign collusion with Russia. The inquiry has netted unrelated guilty pleas for unregistered lobbying, tax fraud, and other crimes, such as lying to investigators from Trump associates and allies.