By Phil Mattingly, CNN

(CNN) -- The White House says it no longer has custody of the documents House Democratic investigators sought to shed light on the inner workings of then President Donald Trump's top aides in the lead up to, and on the day of, the January 6 attack on the US Capitol, according to a letter obtained by CNN.

"It has been the longstanding practice for all White House records to be transferred to the custody of the National Archives and Records Administration at the end of each President's tenure," White House counsel Dana Remus wrote in a letter to House Oversight Committee Chairwoman Carolyn Maloney, a New York Democrat.

"As a result, NARA is the appropriate entity to address your request, and should have any records responsive to your request; we do not have custody of such records at the White House," Remus wrote.

House Democratic committee chairs, on March 25, sent letters requesting documents and communications from before, during and after the attack on the Capitol, from a wide range of entities, including the White House, federal agencies, local law enforcement and the House and Senate sergeants-at-arms.

That request included the National Archives, where Remus said any records responsive to inquiry would now be held.

The request marked a clear ramping up from Democrats into their unilateral investigation into the events that led to the death of five, injured dozens of law enforcement officer and shook the country just two weeks before President Joe Biden was set to be inaugurated.

In the request sent to the White House, specific communications and documents from then-Trump administration employees with any relationship to the events of January 6 were requested -- an area that up to this point has remained largely unknown in the investigations into the attack.

Then-President Trump spent the days leading up the January 6 rally that came before the attack on the Capitol urging his supporters to come to Washington as lawmakers prepared to vote to confirm the electoral count of Biden's victory.

Trump, who spoke at the rally, based his calls for supporters to come to Washington on lies about a stolen election. None of Trump's assertions were ever remotely true, and the efforts of his legal team, and lawyers supporting his campaign, to challenge the results were resoundingly dismissed or defeated.