The insiders claim that Facebook rejected House and Senate Intelligence Committee attempts to get the ads. The site's policy, in line with the Stored Communications Act, requires a warrant before it can hand over search data.
Mueller's spokesperson declined to comment, and Facebook would only say that it continues to "work with the appropriate investigative authorities." Don't expect to learn many specifics, then. All the same, this both highlights the breadth of the election interference investigation and the nature of these sorts of investigations circa 2017. Who'd have thought that social media ad spots could play a substantial role in a case like this?