Washington (CNN) President-elect Donald Trump's transition staff is beginning to move into federal agencies as he prepares to take over the White House, a step former transition officials say is necessary for a smooth transfer of power.

Trump teams for the Defense, State and Justice departments, as well as the National Security Council, are fanning out in Washington.

The Trump transition announced the names of the first wave of staffers who will form the "landing teams" for the four agencies on Friday, a list that included a mix of DC long-timers and newer additions to Trump's orbit.

Those teams will receive briefing materials from the agencies they are assigned to and will have the opportunity to ask questions about how the agency operates. They will take back what they learn to the transition to inform the White House team as well as the future secretary or leader of that agency. Their findings will also help inform the development of policy for the President-elect.

It's a process that has repeated itself in recent presidential transitions and is essential to changing presidents, officials say.

"It's incredibly important," said Deputy Labor Secretary Christopher Lu, the former executive director of President Barack Obama's transition.

"You can only discern so much information about what's happening in a department from publicly available information like a website," Lu added. "Only once you've had a chance to talk to people can you really understand what's going on."

The process of agency landing teams was delayed, in part, by paperwork . The White House needed the transition to submit a list of names of staff they wanted to send into the agencies, so they could be given background checks, and required those individuals to submit to code of conduct and conflict of interest statements.

Lu said the step is necessary for non-government employees to be privy to sensitive government information, especially in the national security field.

Waiting for the landing teams at the agencies are pre-readied briefings, ordered by Obama months ago.

"Those briefings include organizational charts, budget materials, briefings on key agency priorities and areas of responsibility, and other materials describing the essential functions of that agency," the White House says in a fact sheet about the transition. "In addition to the initial briefings, designated employees across the Administration will work closely with their Agency Review Teams in order to facilitate open communication between the outgoing and incoming Administrations."

They should be prepared for a lot of information.

"If our book is 800 pages, I can't even imagine what DoD's is like," Lu said, speaking from the perspective of preparing for the eventual arrival of the Department of Labor team.

Only the first four waves have been named from Trump transition, but others are expected to follow. Pre-election, New Jersey Gov. Chris Christie had been chairing the transition and a number of people had been tapped on standby to lead agency teams. But shortly after Trump and his campaign inner circle reviewed the plan after the election, they replaced Christie with Vice President-elect Mike Pence -- who moved to remove a number of lobbyists from the team along with a few other individuals who were let go for different reasons.

The transition has begun rebuilding since and is now moving down the list of agencies. National security was prioritized, including the announcement Friday of three key nominations: retired Army Lt. Gen. Michael Flynn for national security advisor, Alabama Sen. Jeff Sessions for attorney general and Kansas Rep. Mike Pompeo for CIA director.

Lu says the Obama transition moved more in advance to get staff in agencies. He said upwards of 100 people were sent to the White House before the 2008 election so that they could be vetted and ready to go a few days after the election. Their landing teams were in agencies by the Monday after election, about a week ahead of Trump's first wave.

Both Trump and Hillary Clinton's transitions were given the option to submit names in advance if they so chose.

Despite being highly critical of Trump during the campaign, Obama has welcomed Trump to the White House since the election and been very clear to his staff that a smooth transition is a must.

Lu notes that part of Obama's motivation is how helpful and gracious former President George W. Bush was with his outgoing presidential transition.

"When Obama expressed his appreciation to that team, it was sincere," Lu said. "The message has been sent to us repeatedly over the last six months that regardless of who wins, this administration is going to facilitate a smooth and orderly transition. It's a hallmark of our democracy."