Steve Bannon has been subpoenaed by Robert Mueller:

Stephen K. Bannon, President Trump’s former chief strategist, was subpoenaed last week by the special counsel, Robert S. Mueller III, to testify before a grand jury as part of the investigation into possible links between Mr. Trump’s associates and Russia, according to a person with direct knowledge of the matter.

The move marked the first time Mr. Mueller is known to have used a grand jury subpoena to seek information from a member of Mr. Trump’s inner circle. The special counsel’s office has used subpoenas before to seek information on Mr. Trump’s associates and their possible ties to Russia or other foreign governments.

A second subpoena for Mr. Bannon to testify came from a House panel on Tuesday.

The Mueller subpoena could be a negotiating tactic. Mr. Mueller is likely to allow Mr. Bannon to forgo the grand jury appearance if he agrees to instead be questioned by investigators in the less formal setting of the special counsel’s offices about ties between Mr. Trump’s associates and Russia and about the president’s conduct in office, according to the person, who would not be named discussing the case. But it was not clear why Mr. Mueller treated Mr. Bannon differently from the dozen administration officials who were interviewed in the final months of last year and were never served with a subpoena.

Here’s a thought:  When one is compelled to testify by subpoena, you are indemnified from libel and defamation lawsuits.

If investigators were to want me to testify about someone as litigious as Donald Trump, I would insist on a subpoena to protect myself from vexatious litigation.

One caveat here though, I am an engineer, not a lawyer dammit,^*  so talk to your lawyer before ever talking with cops or prosecutors.

^*I love it when I get to go all Dr. McCoy!