President Trump's former national security adviser John Bolton posted a cryptic message to Twitter on Friday after a two-month absence on the social media platform, telling followers to "stay tuned" for "the backstory."

"Glad to be back on Twitter after more than two months. For the backstory, stay tuned," Bolton wrote.

Before Friday, his most recent tweet was on Sept. 10, when he said he had offered his resignation to Trump. The president previously said he had fired Bolton, who was Trump's third national security adviser.

Trump told the hosts of "Fox & Friends" on Friday morning that he "had a good relationship with John," while adding that they had disagreed at times.

The former White House aide has been mentioned several times during congressional testimony in the House impeachment inquiry into Trump.

Former Russian affairs adviser Fiona Hill said during a public hearing Thursday that Bolton had described Trump's personal lawyer Rudy Giuliani as a time bomb waiting to detonate.

"Ambassador Bolton had looked pained, basically indicated with body language that there was nothing which we could do about it," Hill said of Giuliani's efforts to attack political opponents. "And he then, in the course of that discussion, said that Rudy Giuliani was a hand grenade that was going to blow everyone up."

Hill said in previous testimony that Bolton told her to tell National Security Council legal adviser John Eisenberg "that I am not part of whatever drug deal" other Trump administration officials were "cooking up."

House Democrats have asked Bolton to testify and saw him as a potential star witness, but he has refused to come before Congress, citing Trump's assertion of executive privilege.

House lawmakers decided not to subpoena Bolton after he declined to appear voluntarily, and a House Intelligence Committee official said in a statement that "Mr. Bolton would take us to court if we subpoenaed him."

Bolton's lawyer, Charles J. Cooper, said in a letter to the House's general counsel that he had hoped to determine in court whether a subpoena or executive privilege took precedence.

He also said in the letter that Bolton was involved in "many relevant meetings and conversations that have not yet been discussed" in the probe.

Updated at 9:32 a.m.