Washington (CNN) Did President Donald Trump fire former Veterans Affairs Secretary David Shulkin or did he resign?

If Shulkin, the only holdover from President Barack Obama's administration, was fired, the actions of his acting replacement -- Defense Department official Robert Wilkie -- could be legally challenged as invalid until Trump's permanent pick for the job, White House doctor Ronny Jackson, is approved by the Senate. If Shulkin resigned, Trump can put his own candidate in place to assume the job.

The White House, now seemingly aware of the issue, has maintained that White House chief of staff John Kelly offered Shulkin the chance to resign in a conversation a few hours before Trump tweeted that he was replacing his VA secretary.

Shulkin, on the other hand, has vehemently denied that he resigned, telling multiple outlets that he was flatly fired.

Here is the timeline of what top White House officials and Shulkin have said:

Wednesday, March 28

Shulkin called Kelly on Wednesday morning, according to a Trump administration official, as rumors of his ouster reached a fever pitch in Washington. Trump's view of Shulkin has soured in recent weeks and he was widely seen as the most vulnerable secretary in an already unstable Cabinet. Shulkin, the official said, asked Kelly how his press team should respond to inquiries from reporters who said they were being told that Trump planned to fire him via Twitter at 2 p.m. ET.

Kelly then spoke with the President, who said, yes, he was planning on announcing Jackson as Shulkin's replacement. Trump then asked Kelly how he thought it should be carried out, the official said.

Kelly, according to multiple sources, called Shulkin back and informed him that he was being replaced. It was during that phone call that Kelly offered Shulkin the opportunity to resign, which he did, according to this official.

Trump tweeted at 5:30 p.m. on Wednesday night that he was "pleased to announce that I intend to nominate highly respected Admiral Ronny L. Jackson, MD, as the new Secretary of Veterans Affairs."

He added, "In the interim, Hon. Robert Wilkie of DOD will serve as Acting Secretary. I am thankful for Dr. David Shulkin's service to our country and to our GREAT VETERANS!"

I am pleased to announce that I intend to nominate highly respected Admiral Ronny L. Jackson, MD, as the new Secretary of Veterans Affairs.... — Donald J. Trump (@realDonaldTrump) March 28, 2018

....In the interim, Hon. Robert Wilkie of DOD will serve as Acting Secretary. I am thankful for Dr. David Shulkin's service to our country and to our GREAT VETERANS! — Donald J. Trump (@realDonaldTrump) March 28, 2018

Shortly after Trump's tweet, two sources familiar with the matter told CNN that Kelly called Shulkin before Trump's tweet and informed him that he was being replaced.

A White House official later told CNN that Shulkin was not fired by tweet.

Thursday, March 29

Shulkin, shortly after he was let go, went on a media tour to tell his side of things.

In an interview with NPR , the former VA secretary said he found out "last night" that he was being dismissed but indicated that it wasn't his decision to leave:

NPR: Doctor, I wonder, were you given a reason for why you were let go?

Shulkin: When you're a Cabinet member, you serve at the pleasure of the President, and, you know when the President makes a decision, that's the way that goes. We all understand that taking these jobs.

On the same day Shulkin told MSNBC that he spoke with Trump on the day he was ousted and that they talked about "the progress that I was making, what I needed to do from a policy perspective to make sure we're fixing the issues in VA."

"He was very inquisitive about the things we were working on, making sure we were focused on the job at hand," Shulkin said.

In that interview, Shulkin agreed with the statement that he was "fired," but that Trump made no mention of a plan to terminate him in their conversation.

"Right before (Trump's tweet), the chief of staff Kelly gave me a call which I appreciated, gave me a heads up. But that was much after the phone call," Shulkin said.

Friday, March 30

On Friday, Shulkin told ABC News that Trump "did not mention anything specifically about my position but was really more about the policies that we were putting in place."

Saturday, March 31

Top White House officials, now seemingly aware that it mattered whether Shulkin fired or resigned, made clear in a statement that they believe the VA secretary stepped away from his post willingly.

"Secretary Shulkin resigned from his position as secretary of the Department of Veterans Affairs," White House spokesperson Lindsay Walters said.

Sunday, April 1

JUST WATCHED Shulkin mum on endorsement for Trump's new pick Replay More Videos ... MUST WATCH Shulkin mum on endorsement for Trump's new pick 01:44

In an interview on NBC's Meet the Press on Sunday, Shulkin revealed that during his pre-dismissal call with Trump, he and the President set up a meeting for the following day to talk about the VA.

"Just a few hours before ... we had set up a meeting for the very next day," Shulkin said.

He later said that his dismissal was "was somewhat of a surprise."

Shulkin made that case bluntly on CNN's "State of the Union," telling Jake Tapper that he would "not resign, because I'm committed to making sure this job was seen through to the very end."

"I did not resign," he said.

In response to the interviews, White House officials said on Sunday that they stand by Walters' earlier statement.

"Our statement still stands," said assistant press secretary Ninio Fetalvo.

Asked if there was any concern there may be a legal challenge based on the order of succession -- after the President bypassed Shulkin's deputy to appoint a different acting head of the VA -- the White House would not comment.

Monday, April 2

Shulkin was back on CNN's "New Day" on Monday morning, where he again vehemently denied that he resigned:

CNN's Alisyn Camerota: Were you fired or did you resign?

Shulkin: Well, I came to Washington with the commitment to make our system work better for veterans. That's the commitment that I went to work every day. I continue to feel strongly about that. There was no reason why I would resign.

Camerota: So why not just say you were fired?

Shulkin: I think that's the alternative to resignation. I received a phone call saying the President wanted to make a change. That is certainly his prerogative to do that, and that's what happened.

Camerota: You received a phone call from chief of staff Kelly, who fired you?

Shulkin: General Kelly gave me a heads-up.

Camerota: So the tweet fired you?

Shulkin: Yes.

Shortly after Shulkin was on CNN, Mercedes Schlapp, the White House director of strategic communications, told Fox News that Kelly "Kelly called Shulkin and gave him the opportunity to resign" but that it was Trump who made the decision.

"He wanted a change in the department of Veterans Affairs. He wanted more results coming out of that particular department," Schlapp said. "Which as we know is incredibly bureaucratic. That is why he moved to make this change. That is why he brought in admiral Ronny Jackson.

Pressed on whether Shulkin resigned, Schlapp repeated her past comment.

"General Kelly offered him the opportunity to resign," she said. "At this point the President said it was time to move on in terms of Veterans Affairs. He thanks Secretary Shulkin for his service."