On September 3rd, 2018, President Trump asserted in a tweet that Jeff Sessions, his attorney general, was to blame for bringing charges against two Republican lawmakers ahead of the midterm elections. Trump also claimed that the investigations, which recently resulted in federal indictments, stemmed from the Obama administration.
Politifact ran a fact-check on Trump's assertions, and in the process, spoke to several experts.
In regards to the case against Duncan Hunter, Professor Andy Leipold said, "A grand jury usually will work on a lot of matters, and even if this grand jury started in September 2016, it might have been working on other things until the last several months."
Full story at politifact.com.