TIMELINE: When key figures found out about Lloyd Austin's hospitalization

Defense Secretary Lloyd Austin's hidden hospitalization and intensive care stay is roiling the Pentagon in controversy over its handling of his condition and their communication with the White House, commanders, Congress and the press. The Pentagon is now revealing Austin was transported to the hospital via ambulance on the evening of New Year's Day, Monday, three days before the president was informed.

Also in the three-day period between the Jan. 1 and Jan. 4, the Pentagon's No. 2 official, who would step in to Austin's role in the event of a vacancy, was unaware that the secretary was in intensive care and was on vacation in Puerto Rico.

Here is a look at the timeline of events so far:

FRIDAY, DEC. 22

Secretary Austin undergoes an elective medical procedure at Walter Reed National Military Medical Center in Bethesda, Maryland, and stays overnight. Deputy Secretary Kathleen Hicks, the No. 2 at the Pentagon, is given temporary authority, but isn't told why.

SATURDAY, DEC. 23

Austin returns home after his procedure.

SATURDAY, DEC. 30 - SUNDAY, DEC. 31

At some point over the weekend before New Year's Day, Secretary of State Antony Blinken said he spoke with Austin, but a medical issue didn't come up.

MONDAY, JAN. 1

Per National Security Council spokesman John Kirby, Austin huddles via conference call with the president, Blinken, and National Security Advisor Jake Sullivan to discuss "operations in the Middle East."

"On the morning of the first of January, [Secretary Austin], as well as Secretary Blinken, Jake [Sullivan], other relevant officials did have a secure conference call with the President. This was regarding operations in the Middle East. And so, there was -- there was contact on the first of January," he said.

On the evening of Jan. 1, Austin experiences severe pain and is transported by ambulance to Walter Reed National Military Medical Center where he's admitted to the Intensive Care Unit.

TUESDAY, JAN. 2

Gen. C.Q. Brown, the chairman of the Joint Chiefs of Staff, is informed that Austin had been hospitalized the night prior.

Austin's Chief of Staff Kelly Magsamen, who is sick with the flu, and his senior military aide are made aware. Chris Meagher, the assistant secretary for public affairs, tells Maj. Gen. Pat Ryder, the Pentagon press secretary, that Austin is in the hospital but doesn't reveal details of his condition.

Deputy Secretary Hicks, vacationing in Puerto Rico, assumes some secretary-level duties, which the Pentagon says is standard procedure, but she is unaware of Austin's condition. She's equipped with secure communications on her holiday, but no one has told her she's the top authority at the Pentagon.

THURSDAY, JAN. 4

At about 4 a.m. eastern time, the U.S. carries out an airstrike in Baghdad that kills a leader of an Iranian-backed Iraqi militia group and his associate. The Pentagon says the strike was approved by Austin before he was hospitalized.

Per Kirby, the White House learns in the afternoon that Austin is in intensive care. The notification comes from Magsamen to Sullivan, Ryder said. Magsamen also tells Hicks, who had been delegated certain operational authorities up to that point.

Ryder said Masamen was "unable to make notifications before then" because she was ill.

Ryder held his regular Pentagon press briefing with reporters on Jan. 4, but didn't disclose any details about Austin's hospitalization.

FRIDAY, JAN. 5

Austin resumes his duties as secretary from the hospital in a private room. The Pentagon hasn't shared when he was released from intensive care.

In the afternoon, the secretaries of the Army, Navy and Air Force, as well as the military chiefs of those services, receive their first notifications that Austin had been hospitalized.

Also in the afternoon, the Pentagon gives notice to Congress and shortly after releases a public statement.