Special counsel Robert Mueller appears to be zeroing in on Roger Stone, a longtime ally of President Donald Trump who briefly served on Trump’s presidential campaign. So far, prosecutors have secured testimony from at least 13 Stone associates, including some who were subpoenaed.

Mueller is investigating Russian interference in the 2016 campaign and potential collusion between Trump’s team and Russia. Stone vehemently denies participating in any conspiracy with the Russians and says the Mueller investigation is a sham designed to hurt Trump.

Here is a list of the Stone associates under scrutiny and their interactions with Mueller.