WASHINGTON — President Trump’s chief of staff and a White House lawyer representing the president in the Russia investigation were present on Thursday at the start of two classified meetings requested by members of Congress to review sensitive material about the F.B.I.’s use of an informant in the inquiry.

The two men left both meetings after sharing introductory remarks “to relay the president’s desire for as much openness as possible under the law” and before officials began to brief the lawmakers, the White House said in a statement.

But the presence of John F. Kelly, the chief of staff, and Emmet T. Flood, the president’s lawyer, infuriated Democrats, and legal experts said their visit, at the least, could give off the appearance that the White House abused its authority to gain insight into an investigation that implicates the president.

The president’s legal team was unapologetic. “We are certainly entitled to know” what information the government has on the F.B.I. informant, Rudolph W. Giuliani, another lawyer representing Mr. Trump in the investigation, said in an interview. The meeting “cuts off a long subpoena,” he said, referring to a legal fight for the information.