Former National Security Advisor to President Donald Trump resigned in September, but he's been back in the news the past few weeks after testimony from diplomat Dr. Fiona Hill in the impeachment inquiry against Trump.

Trump's personal lawyer Rudy Giuliani, Ambassador to the E.U. Gordon Sondland, and White House Chief of Staff Mick Mulvaney (among others) conducted a pressure campaign on Ukrainian officials to announce an investigation into the President's political rival, former Vice President Joe Biden.

Dr. Fiona Hill testified in a closed-door hearing that Bolton said to her:

“I am not part of whatever drug deal Sondland and Mulvaney are cooking up.”

Hill also testified that Bolton said of the President's personal lawyer:

“Giuliani’s a hand grenade that’s gonna blow everybody up.”

This revelations along with other testimony implicating Bolton have lead to growing calls for Bolton to testify before the House Intelligence Committee overseeing impeachment.

Bolton hasn't responded, until now.

After months of silence on his Twitter account, Bolton suddenly began tweeting again this morning—one day after Dr. Hill's public testimony--with a decidedly cryptic message.

Which he soon began elaborating on:

According to Bolton, the White House denied him access to his Twitter. He further implied that the White House was scared of what he might say and asserted that he hadn't gone into hiding.

But it's not Bolton's silence on Twitter that's concerning Americans. It's his silence throughout the impeachment proceedings.

Twitter users called for him to testify.

Bolton has previously said he wouldn't testify without being subpoenaed by Congress—a subpoena Congress has been hesitant to deliver for fear of it being contested by the White House and subsequently held up in court.

"Testify" began trending on Twitter shortly after Bolton's tweets.