An investigation into the handling of classified information by the White House and the team that oversaw Donald Trump’s transition to the presidency has been launched by the House Oversight and Reform Committee following what it describes as a litany of “grave breaches of national security at the highest levels.”

Committee chairman Rep. Elijah Cummings (D-Md.), in a letter sent to the White House on Wednesday, outlined concerns about the handling of sensitive information dating back to Trump’s 2016 election.

“For the past two years, I have sought information with other committee members about a series of extremely troubling incidents regarding the security clearances of some of President Trump’s top aides, but the White House has refused to provide the information we requested, often ignoring our requests completely,” Cummings said.

ASSOCIATED PRESS Rep. Elijah Cummings, chairman of the House Committee on Oversight and Reform, sent a letter to the White House on Wednesday detailing concerns about the handling of classified information.

The probe is one of the first to target Trump’s administration since Democrats won the House majority in November’s elections.

Under Cummings, who took the panel’s helm when the new Congress convened early this month, the committee has also arranged for Commerce Secretary Wilbur Ross to testify before it about a controversial change to the 2020 census and for Trump’s former personal attorney, Michael Cohen, to answer questions under oath about his past dealings with the president.

Ross is expected to testify before the committee on March 14. Cohen had been scheduled to testify on Feb. 7, but on Wednesday he postponed his appearance, citing ongoing threats against his family from Trump and the president’s personal attorney, Rudy Giuliani.

According to Cummings’ letter, the investigation will focus on why previously established procedures for safeguarding classified information were not followed and how much classified information was wrongly provided to officials who did not meet the required standards.

The White House did not immediately respond to a request for comment.

ASSOCIATED PRESS President Donald Trump has been criticized over his administration granting classified security clearances to a number of officials, including Jared Kushner, his son-in-law and senior adviser.

It will also examine how to “remedy the flaws in current White House systems and practices” and investigate why the administration has refused to provide Congress information about its security clearance process, as required under federal law.

Cummings noted that former White House chief of staff John Kelly has spoken of major “shortcomings” with the White House’s security clearance process, telling his colleagues in a letter last year: “We should — and in the future, must — do better.”

Cummings’ letter, in addition to providing a list of past examples of concern, requests a number of documents that would cover the period from November 2016 to the present. Those documents are requested by Feb. 6.